Enable auto-completion for Homebrew in bash

While reading the Homebrew documentation I noticed that there was a
convenient way to enable completion for the `brew` command in bash.
Remove shortcut for resetting window number

Now that tmux 1.8+ supports the `renumber-windows` option, this binding
is no longer used.
Enable pbcopy/pbpaste in tmux

Since `tmux` starts its server using `daemon`, it loses connection to
the user's namespace, which makes it unable to access the user's
clipboard.

This is solvable by using a tool called `reattach-to-user-namespace`,
with is installable via Homebrew. By changing the `default-command` of
`tmux` to check for the existence of the program and starting up a shell
using it, we can reattach the shell to the user namespace.

As a fallback, if `reattach-to-user-namespace` not installed we just
open the user's shell based on `$SHELL`.
WIP