git: set gitconfig push.default to `simple`

This is a better default, and the default in Git 2.0

Fix gh-21
bash: prompt improvements

Port over @alrra's improvements to prompt performance and code
organization.
bash: use alrra's cleanup of bash_profile
Fix homebrew error checking for rsync

Homebrew was complaining when looking for the rsync formula because it's
specified with a URL rather than a known package name. Move the rsync
installation from the main set of Homebrew install checks and steps.

Fix gh-19
Merge branch 'master' of github.com:necolas/dotfiles