Implement fix-permissions verb.

 - Recursively change ownership of ROS HOME to current user.
 - Fixes: https://github.com/ros-infrastructure/rosdep/issues/332
Fix error message for cache permission errors.

 - Tell user to use `sudo rosdep fix-permissions`
Merge pull request #333 from NikolausDemmel/fix-permissions

Implement fix-permissions verb.