Fixed GlReduction, now working reliably for 2D and for 3D images. Added test pipeline GlReduction test.

* Extended GeometryRendererDemo to render two different geometries and combine the renderings using the RenderTargetCompositor
* Fixed point size in GeometryRenderer

* Introducing MultiIndexedGeometry data type
* Introducing GeometryDataFactory::createTeapot()

 * Supports different render modes: GL_POINTS, GL_LINES, GL_LINE_STRIP, GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_POLYGON
 * Supports optional wireframe rendering using Single-pass Wireframe Rendering technique
 * Supports calculation of per-face normals, if not present in geometry
 * Fixed shading

Refactoring AbstractProcessor::process() for clearer semantics and better and more uniform handling of invalidation levels:
 * AbstractProcessor::process() now calls updateShader(), updateProperties() and/or updateResult() with respect to the current invalidation level
 * each processor shall no longer override process() but the updateXYZ() methods, at minimum updateResult()
 * AbstractProcessor::process() takes care of (un)locking the processor itself (no need to do this from the outside anymore)

Further implicit changes:
 * Removed redundant HasPropertyCollection::updateProperties()

Completely moved the image data handle and histogram computation from the TF to the TF property. Histogram computation is now thread safe.

Started refactoring the TransferFunctionProperty: The optional ImageData handle will be stored in the TF property instead of the TF itself. Furthermore, the property now supports automatically fitting the TF window to the image data.

 * Introducing campvis::Tensor2<T> type
 * Updated MhdImageReader to support TensorImages

* fixed VolumeExplorer/VolumeRenderer not correctly updating the proxy geometry due to missing locks and thus skipped invalidations
* adjusted VolumeExplorerDemo image URL

Conflicts:
	modules/io/processors/abstractimagereader.cpp
	modules/io/processors/abstractimagereader.h
	modules/io/processors/csvdimagereader.cpp
	modules/io/processors/csvdimagereader.h
	modules/io/processors/genericimagereader.cpp
	modules/io/processors/genericimagereader.h
	modules/io/processors/ltfimagereader.cpp
	modules/io/processors/ltfimagereader.h
	modules/io/processors/mhdimagereader.cpp
	modules/io/processors/mhdimagereader.h
	modules/io/processors/rawimagereader.cpp
	modules/io/processors/rawimagereader.h
	modules/io/processors/vtkimagereader.cpp
	modules/io/processors/vtkimagereader.h

Loading can be done with genericImageReader, however, assertion fails for sliders. thread ownership issues

*IMPORTANT FIX* Fixes possible crashes/segfaults when converting OpenGL image representations to local image representations:
 * ImageRepresentationLocal::tryConvert() does not deadlock anymore when called from OpenGL context and converting from GL representation
 * Fixed possible double deletion after converting from OpenGL to local representation due to ambiguous pointer ownership
 * fixed possible out of bounds array write in tgt::Texture::downloadTextureToBuffer()

fixed PropertyCollectionWidget changing visibility of property widgets from non-Qt thread (now using Qt signals).

Loading can be done with genericImageReader, however, assertion fails for sliders. thread ownership issues

Added smallHeart.mhd sample volume to repository, all demo pipelines in vis module refer to this file. Hence, all vis pipelines should work out of the box from now.

Closes Issue #16: Geometry TF editor thread-safety: Reviewed the code + extensive stress testing - fixed a recursive mutex acquisition.

* Moving to Apache 2.0 license
* Updated AUTHORS.txt