adds gradients for RigidBodyThrust.  user gradients work for quadrotor now.
dircol example works now.  exits w/ 41, but the output looks good.  will have to look more carefully soon
resolved error 41.  forgot to uncomment my cost fun gradients.  ;)
addressed Benoit's comment, added a (very) little documentation
Merge pull request #173 from RussTedrake/thrustGradients

Thrust gradients and quadrotor dircol example
Merge pull request #175 from RussTedrake/thrustGradients

addressed Benoit's comment, added a (very) little documentation