Add technical reference for the position IK solver

[MatthewPeterKelly]

The position IK solver works by using gradient descent combined with a barrier function. It would be great if we had some sort of technical reference to go over this math in detail.

[rethink-forrest]

Two quick notes:

1. The code pull-request for this is Decaying barrier function to handle joint limits #33
2. It seems like I based the decaying logarithmic barrier function partial from these notes:
   http://s-mat-pcs.oulu.fi/~keba/Optimointi/OP_penalty_engl.pdf