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It has been superceded by a `.local` suffix convention.
Git push to current, not upstream

The primary use case for me is to `git push staging` and `git push
production` from the master branch using our typical git workflow:

https://github.com/thoughtbot/guides/tree/master/protocol#deploy
Make the migrate alias faster

This was loading up the environment four times when it really only needs
to happen twice.
Remove unused vim plugins

* haml, html and markdown highlight still work without these plugins
* treetop and textile are likely very situational
* unsure if anyone is using greplace
Remove old ignores

* sass gets compiled to `tmp/sass-cache` with Rails
* vendor/bundle_gems seems like an arbitrary vendor directory
* vim/.netrwhist is for a plugin we no longer use
List local files available to override
fix relative .aliases.local path
Add syntax highlighting to certain file types

* Syntax highlight `Appraisals` as Ruby. Used especially for our open
  source libraries.
* Syntax highlight rackup (`config.ru`) files as Ruby. Used in all our
  Ruby web apps. Rails apps are rarely touched but some of the Sinatra
  and Middleman apps are occasionally edited.
* Syntax highlight JSON files as JavaScript. Used in our Trail Map.
Remove redundant syntax highlighting

Newer versions of vim (such as 7.3.923) syntax highlight Rackup and JSON
files correctly. In the interest of keeping the dotfiles slim and our
machines up-to-date, I think the solution is to remove these from
dotfiles and install a newer version of vim in thoughtbot/laptop:

https://github.com/thoughtbot/laptop/pull/118