Implement chruby for Fish Shell

This is my first stab at getting chruby to run in the Fish shell. This was a request at https://twitter.com/nirvdrum/status/236594671986565121 which I also really needed to finally switch from RVM/Rbenv.

It works for me™ but still needs tests to iron out all the possible bugs.

- [x] rewrite `chruby.sh` in Fish
- [x] rewrite `auto.sh` in Fish
- [ ] add tests
- [ ] update README
- [ ] …

Only problem: I haven't written a single line of Fish test yet, and am not even sure how easy it would be without an equivalent of shUnit2. Although there are some examples at https://github.com/fish-shell/fish-shell/tree/master/tests, but I haven't looked into those yet.

Any thoughts on this?