Minor change
Merge branch 'master' of https://github.com/FeiZhu/Physika

Conflicts:
	Physika_Src/Physika_Core/Matrices/sparse_matrix.cpp
Add functions to change methods in RigigBodyDriver
Merge branch 'master' of https://github.com/FeiZhu/Physika
Modify the interface of rigid_body_driver_utility
Merge branch 'master' of https://github.com/FeiZhu/Physika
Merge branch 'master' of https://github.com/FeiZhu/Physika
Add compressed matrix