Add Cython binding for zmq_msg_gets
Add tests for message properties in test_security
Return None instead of bytes() in Frame.gets
Add backwards compatability for zmq_msg_gets and Frame.gets
Skip Frame metadata test if unsupported version
Merge Frame.gets into Frame.get
Skip msg metadata tests on tests.PYPY
finish up support for zmq_msg_gets

- use str/unicode, not bytes (zmq msg properties are defined as utf8)
- support gets with Frame['key']
Merge pull request #542

adds support for `zmq_msg_gets` in Frame.get, and `Frame['property']` dict-access.

closes #542
closes #575