Drake gradient util

Pull request for C++ gradient utilities. I put general purpose functions like {get,set}SubMatrixGradient, transposeGrad, etc. in drakeGradientUtil, and functions that compute gradients of geometrical things in drakeGeometryUtil.
i thought a bit about this, and it is actually hard to do the right thing in the cmake file (since the eigen libraries might have been include via pkg-config, or by the findpackage command).  

i think by far the better solution is to zap this line, and refer the the headers as
`#include<Eigen/unsupported/blah>` in the source files.  That should work, right?