CFFI capitalization
Merge pull request #380 from minrk/caps

CFFI capitalization
Merge pull request #381 from minrk/pollperf

minor performance tweaks to poller

Give a ~2x improvement for the simple case of a single Socket on my laptop (2.5 GHz i5, OS X 10.8.4).

- before: around 55k msgs/sec
- after: around 110k msgs/sec
Remove unhelpful GIL releases

We used to release the GIL on all libzmq API calls, but many of these calls are not blocking. Since there is a finite cost to GIL transactions, I have removed the GIL releases that are not on blocking calls.

Simple testing shows a notable improvement in performance for multithreaded apps (simultaneous `while True: send/recv` threads).