Bug fixes and can now specify background color

- Added ability to specify background colors.
- A more helpful error message is given when trying to create a k-d tree from an
empty list.
- Scene.calculate_color can now be called from Python code.
- Added example of usage without Pygame to documentation.
- Fixed wavefront_obj.load_obj not working with Python 3.
- No longer tries to erroneously link with SDL
- When compiling with Clang, no longer assumes __GNUC_MINOR__ corresponds to the
standard library version. This lets Clang build with versions of libstdc++
greater than 4.7, but not 4.7 itself.
- Fixed bug preventing the detection of superfluous keyword arguments in
functions that have default values for parameters.
- CallbackRenderer now correctly reacquires write lock even when calling the
callback function causes an exception.
- Minor code refactoring done.
- Changed status to Beta.