Add performance tests

[ColeDCrawford]

PR #36 adds packratting for pyparser to improve performance. I think it would be great to merge in that feature, but also useful to take some benchmarks for before and after. These can be done as part of the CI, though I don't think we necessarily need them to hit specific metrics for the CI to pass.

[aweakley]

The benchmark graphs are appearing here now https://ixc.github.io/python-edtf/dev/bench/ The docs say it's not safe to allow updating your gh-pages on pull-request, so I've excluded PRs which is a bit of a shame. I'll try to break it into two steps.

[aweakley]

This does seem to have made quite a difference in some cases: