GitLab

Add helper function to get rvm head. Add helper function to link zsh
completion that comes with rvm into om-my-zsh plugin directory, but don't
overwrite the completion that comes with oh-my-zsh (oh-my-zsh's completion is
better, but I want to be able to compare).

Add alias to list installed rubies. Add alias to list gemsets in active ruby.

Add helper functions to switch gemsets on ruby-1.8.7-p334 and ruby-1.9.2-p180.
Add completion definitions for helper functions.

I've changed it to use `view` instead of `vim -R` to make it hopefully
more editor-agnostic.

In detail:
gup - now completes with remotes like git fetch would
gdv - got git diff completions
gc/gca - now zsh will suggest files in staging area
gba - suggestions as for git branch