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.
No, as in

```javascript
    var commitRows = this.props.commits.map(function(commit, idx) {
        return <CommitRow
            key={commit.hex}
            commit={commit}
            idx={idx}
            selected={idx == this.state.selectedCommitIdx}
            handleSelected={this.handleSelected} />;
      }.bind(this));
    return <table className="commits"><tbody>
      {commitRows}
    </tbody></table>;
```

Then you don't have so much logic & markup mixing.