Add benchmarks with real data

We have about 10 MB of data in here now, and some benchmarks that use it. You can try it like this:

```
make -C data  # Only needs to be run once
asv run -qe  # Check for problems
asv run v0.2.1..HEAD  # Benchmark all working versions; takes ~1 hour
```

We have not yet made any changes to `master` that meaningfully affect performance. So the results pages look something like this:

![results](https://cloud.githubusercontent.com/assets/2985325/4022263/708c45a4-2b26-11e4-935f-2c4774910efb.png)

This needs some refinement, but it seems like a good foundation. The nice thing about `asv` is that you can come up with a good improvement for the benchmarks, throw out all your old results, and have new ones within an hour or so.