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.
As is, React is using the array index as a key. If two objects have the same key, it assumes they're the same. So if you ever reorder the list or add an item to the top, you could get an inconsistent UI (or maybe just more re-rendering than is necessary, I'm not sure). I don't think it's so bad to reach in and get `commit.hex`!