Merge pull request #13 from ericye16/master

small fix -- fixes rref call
Update matrix.cpp

Fixed determinant bug and added a quickDet() function to much more efficiently calculate determinants using RREF
Update matrix.h
Update matrix.cpp

Few more bug fixes and added invertible() function
Update matrix.cpp

Updated print function, added nullspace function, added simple build function (which has a bug in it).
Update matrix.h
Update matrix.h
Update matrix.cpp
Update matrix.cpp