Add a git pull check

My take on #6. Not sure if this is how I want it or if it even belongs here. I tried to decouple it from the prompt, but it's unnecessary hard with zsh, so I went with this for now.

For anyone looking. This will check if there's anything to `git pull`. Useful so you don't start working on a lot of changes that might already conflict.

![pure-git-pull-check](https://f.cloud.github.com/assets/170270/1142674/094860ae-1ce4-11e3-8cfc-6335716221b9.gif)

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// @hemanth @pbrisbin