Added shell functions for fish shell

Hello, this is a commit to provide a version of the various shell functions found in ros bash to the fish shell.  Specifically, this is for the forked version under current development on github rather than the legacy version in the Debian/Ubuntu repositories.  I don't know anyone currently using that version.

I've duplicated all the functions, but none of the auto completions in this patch.  rospd works slightly differently but mirrors the way in which fish does pushing directories.

More info about fish:
https://en.wikipedia.org/wiki/Fish_shell
https://github.com/fish-shell/fish-shell