Add vim-repeat / InsertChar to vim bundles

This, along with the associated mapping, lets me insert *one* character
with tab.

This commit also includes a NERDTree key binding that reminds me of
Sublime Text (^K, ^B).
Start using reattach-to-user-namespace to fix tmux

More specifically, this fixes tmux and pbpaste / pbcopy on Mac OS X.
Unfortunately, this is super non-portable and will almost certainly have
to be changed to maintain compatibility with my Linux machine.