adds comments, minor functionality, and an example
dircol example
moved obstacle field code from util to airplane2d directory, since that is the only example using it (and it's been trumped by the bullet objects)
updates trajectory optimization examples (airplane2d now using new traj opt interface).
changed solveTraj in DircolTrajectoryOptimization to output utraj before xtraj.  (sorry for the change, but it's definitely the right order since you might want utraj without xtraj but not vice versa)
updated glider and dubins to new trajectory optimization interface.
(runDircolPerchingURDF was essentially a non-test since the number of break points was 2, and it didn't work for me on first try with more, so i've moved it to dev)
converted remaining code to new trajectory optimization interface.
note: did not test lqr tree code (since it's only called from dev examples)
zaps old trajectoryOptimization interface.  resolves #321.
edits based on Hongkai's code review
edits based on Andres' code review
revert output argument order change in solveTraj
take geval back out of direct traj opt methods
Merge pull request #482 from RussTedrake/traj_opt_notes

cleanup trajectory optimization examples (now everything uses the new interface)