Added C4251 to the ignore list for MSVC warnings, occuring due to missing template instantiations of the standard library used in public interfaces of tgt and campvis code. Even though articles such as <http://support.microsoft.com/kb/168958/en-us> provide a clean solution to fix these warnings, I decided that exporting bazillions of std library templates is not worth the effort. As we do not plan to ship TGT without the rest to a variety of systems, the missing exports won't be a problem.
BTW: The original tgt library build scripts also deactivate the warning. :)

closes #376