Bring CPR fork changes upstream

[efernandez]

Bring changes since June 8, 2015.

• First PR is in, waiting on @bmagyar 's review: Fix odometry twist computation #294

• Next PR should take the 5 commits from https://github.com/clearpathrobotics/ros_controllers/pull/3/commits, which will bring the following features:

1. Separate wheel radii
2. Covariance computation
3. Roslinting and warning fixes
4. New unit tests (mainly for the new covariance computation functionality)

• TBD (I'd update this as I create new PR and we go forward, since all commits are on top of each other, and things need to be reviewed and merged sequentially).

FYI @bmagyar @artivis