Unittest Prototype added

A first version of a unittest framework, based on UnitTest++, has been added to the SCons build.

So far it only supports text-based tests, graphical tests to be added soon. But there is a lot of things that can be tested on a text base, so feel free to write some tests. See source:trunk/Source/Base/Base/OSGVectorTest.cpp for some simple examples. The test executables are built in build.*/type-*/unittest.