Fixed namespace gtr -> grt, added Scene.getName()

Corrected misspelled namespace (gtr to grt [GPU Ray Tracer]).

Added getName() function to Scene class that obtains the string given to
the Scene constructor.
Initial Unit Test Case Framework

Rework SConstruct file to build a library that is now used by the main
application (src/main.cpp) and by the Unit tests (test/test_suite.cpp).

Unit test example added for Scene class; specifically getName function.

Unit testing via the Google Test framework.