Feature/eigen kinematics property tree exports

I've exported the xFromPropertyTree functions to python.

For this, it was also necessary to add some includes to the C++ headers, where these functions are originally defined. I pondered if I should make this a separate pull-request, but decided against it because the exports depend on this change. I felt that having two pull requests would be much more confusing.

This pull request is based off the "fuerte-devel" branch. I think having this change would make sense both there and in master. I'm unsure about the correct way to proceed, though.