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
Add Git credentials.