PERF: Cython optimizations for period module, round one
Each commit passes `test_fast.sh`, so performance can be measured between commits.

I can break any of these commits into separate PRs if needed.

vbench results

https://gist.github.com/blbradley/3b0706bb2efe908c7026
 I removed one commit because I thought it was a bad decision. I don't think it will affect the performance results too much.
 Looks good to me.

You'll need to add types if you want significant speedups for things like the `Period` constructor, though.
 Current build failure is a problem with Travis CI and PyPI package resolution. Happening with other builds too.
 @blbradley thanks for the report, I opened a conda issue for that: https://github.com/conda/conda/issues/1166
 can you post an updated vbench?
 @blbradley The build did pass when I restarted it (after conda deployed their fix)
 @shoyer fyi, I turned off `ssl_verify` to fix this (subsequently conda DID fix requests though)
 lgtm, @shoyer ?
 Yes, looks good to me!
