Streamed response support

Instance-level reimplementation of [#15](https://github.com/michaelhelmick/python-mailsnake/pull/15)

Adds support for streamed responses (potentially very large ones from the export API, for example) by modifying `MailSnake.__init__()` to accept a new kwarg `requests_opts` that is passed through to `requests.post()` on each request. If `'stream': True` or `'prefetch': False` is present in `requests_opts`, `MailSnake.call()` returns a generator that yields each (deserialized) line as it arrives.

Tested in Python 2.7.3 and 3.2.3 against Requests 0.14.2 and 1.0.3.