Added shell functions for fish shell #2

Continuation of this pull request: https://github.com/ros/ros/pull/16

Sorry for taking so long to get back to this, there was a lot of work to get done on other things before heading off on vacation and I'm afraid this fell to the wayside.  I think this addresses all the concerns you had.

The version of fish this is made for is finally going into mainline Ubuntu with 13.10.
http://packages.ubuntu.com/saucy/fish