Move code to /src folder
Add getCubeNumber() method to support CUBE_MAT_NUMBER ordering of matrices
Auto-detect OMX vs. Cube, and require CUBE_MAT_NUMBER attr for converting OMX->Cube

Sorry this should have been two separate checkins.
Smarter file extension logic.
Make sure input file exists/openable
Update readme
Merge branch 'cube_mat_num'