User gradients for wing elements (and other manipulator functions)

This should work but also deactivates mex on line 5 of manipulatorDynamics. This is due to the fact that HandCmex doesn't return the same gradients as the matlab code does. I think a code review of HandCmex and manipulatorDynamics would really help - I only had to add two lines to the C file so it is mostly Russ's code. 

<b>Note:</b> The most recent commit in this pull request removes code that can actually be used to test the accuracy of those gradients. The code also reveals a possible bug in geval so let me know if you want to use it (you will need the fix to geval for that).