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.
One advantage of not using custom JSON serialization is that you can make this:
```javascript
<script>var commits = {{commits|tojson}};</script>
```

And ditch the `autoescape false` block.