add /commits page, with simple highlighting

This is theoretically the start of some richer, web-based ways of interacting with your local git repository.

Example screen:

![](http://f.cl.ly/items/1Q2r3E3Z3b3j280L0w2U/Screen%20Shot%202014-09-20%20at%203.19.06%20AM.png)

open questions:
* do I need to declare the new py deps, `json` and `pygit2` (which depends on `libgit2`) somewhere?
    * `pip install cffi libgit2 pygit2 json` should do work.
I'm just grabbing commits from git and then serializing the fields I currently care about to json. Agree that we could formalize some intermediate state with the fields we care about, but it doesn't really seem necessary at this point. Routing through this json encoder actually can give us a uniform set of fields across some of these git classes that will keep things pretty simple on the server side for a while..