Add event perf test
- do perf tests twice to avoid warm-up/compilation costs
- Make LogStash::Event#[] and #[]= use cached HashEval compilation
  This improves performance of field lookups dramatically

    100,000 iterations looking up "[j][k1]", durations:
      ruby          | before   | after
      --------------+----------+-------
      JRuby 1.7.4   | 1.347    | 0.373
      MRI 2.0.0     | 1.83     | 0.296

   (before == before this commit, after == after this commit)

Woot!
Tests are still passing.