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
Isolate credits to README

Per @mikeburns' suggestion.
Use Silver Searcher as CtrlP backend

* Use `ag` as the find method for listing files in CtrlP Vim plugin
* ag indexes and searches far faster than the native engine
* ag also respects .gitignore files on on a per-directory basis, avoiding duplication
Turn off banner at login

When logging into a new shell, users see a banner such as:

    Last login: Sun Jun 23 11:41:42 on ttys007

Adding an empty ~/.hushlogin file turns that banner off:

http://kb.iu.edu/data/acdd.html
Remove double escaped spaces in vimrc

* Bash users experience errors when using the double-escaped spaces for the CtrlP-SilverSearcher integration.

Thanks to @adamyonk for raising this issue
Include gitconfig.local file for personal changes

* assumes same directory existance
* allows for easier addition of personal configs
Include local aliases file

* `.aliases.local` should be used to add personal aliases
Source ~/.zshrc.local if it exists

Follows the pattern of:

* ~/.aliases.local in ce7ad49
* ~/.gitconfig.local in 8e141fe
* ~/.vimrc.local in a666267

Use consistent:

* `source`ing style.
* commenting style.

I am using ~/.zshrc.local as a home for `eval "$(rbenv init -)"`.
Remove "DO NOT EDIT BELOW THIS LINE" convention

It has been superceded by a `.local` suffix convention.
updated for my tastes
mappings for tmux navigator, and clearing search