Feature: support ProcessPoolExecutor (Python >= 3.3 required)

[asfaltboy]

Change Summary

• include tests, some of which are skipped if on unsupported python version
• use functools.partial instead of directly wrapping the request method, regardless of executor used - avoid code branching
• use functools.partial(Session.request, self) instead of super(FuturesSession, self).request for pickling, again no code branching

  Notes / Limitations

• Python version 3.4 (actually >= 3.3.5) is required in order for us to pickle methods, due to python issue 9276. Though we may be able to use copy_reg to add this support to Python 2.
• Python 3.5 is required to "create a session on demand", with lower versions an existing session is required. Probably due to one of the numerous pickle related issues fixed in 3.5.
• Adding custom attributes to the response object in the callback is supported in ProcessPoolExecutor but only when the callback also modifies __attrs__ so it gets pickled and sent to parent process (see an example for that in the tests).
• As mentioned, the background_callback MUST be a top-level (or otherwise importable) object. Additionally, since an instance of FuturesSession is passed to the callback (as self), it too must be importable, otherwise pickle will complain.

  Conclusions

This relatively simple implementation fixes issue #11, however due to numerous caveats, I'd suggest we recommend it be used in Python 3.5 and above and explain the limitation of earlier versions. If earlier versions (like Python 2) are really required by someone they may need to implement a manual fix to the pickle implementation (e.g using copy_reg).

Personally, this PR solves my issue but I have yet to test it on production, hopefully, will do so in the near future.

[ross]

Sorry for the slow response. Super busy at work as of late. Looks pretty good. Few small comments inline.

[asfaltboy]

No problemo. I added an exception, some documentation and updated tests. Unfortunately, the README.rst does not render so well in GitHub, so feel free to style the doc to your liking.

[ross]

Thanks.