Extract dev/staging/prod binaries to parity gem

https://github.com/croaky/parity
Add mappings to switch windows more succinctly
Remove reattach-to-user-namespace

This is Mac OS X-specific. Including this line in the dotfiles causes
`tmux` and `tmux new -s new-session` to break with `[exited]`.

I personally don't do copy-pasteable work inside tmux anymore.

I'm sure this is valuable to some of the thoughtbot vim users but I'm
thinking it better belongs above `DO NOT EDIT BELOW THIS LINE` as custom
configuration for those users.
Consolidate autocommands into the augroup

This makes it so resourcing the vimrc file doesn't duplicate the
auto commands.
Add shortcut to toggle between two most recent panes

* Mimics GNU screen's ctrl+a a
`git up` alias and `git up -i` documentation
Change default color scheme