basic performance tests

This scratches an itch I've had for a while - it provides a (for now, rather basic) way to test performance across different versions of Ractive.

Alarmingly, it looks like we've got slower since 0.6.1, at least in the tests that are included so far! Without this, we might have shipped that performance regression - now we have a chance to fix it.

![screen shot 2014-12-09 at 4 25 05 pm](https://cloud.githubusercontent.com/assets/1162160/5366200/fee4b3fc-7fbf-11e4-9722-e9ed42f3fbc7.png)

The README in the `perf` folder explains how to set it up and add more tests. Contributions of all kinds welcome, as ever.