Add the possibility to specify the nanomsg implementation

[evadot]

We can now use either nanomsg-python or nnpy. For nanomsg we can now specify the wrapper to use. As nnpy isn't on PyPi yet (there is an issue about it) it is not in the install_requires.

[walkr]

A few thoughts:

• I personally don't really like doing imports and other magic in the __init__ as it's not very clean, nor modular, nor easily extensible. What if we'll discover another new python nanomsg library. Then what? Plus, it cannot be easily tested.
• A service/client accepts a socket as an option, so you can provide your own socket in case you need to bypass nanomsg. (It won't quite work for pub/sub).
• nnpy does not support python 3.4+. I do remember having tested both nnpy and nanomsg at the time this repo was brought into existence, and the latter emerged victoriously as a better option. I don't recall specifically the exact advantages, but nanomsg was chosen as the better option.
• I would preferably not want to support every single option under the sun, just for the sake of providing more options, at the expense of complexity.
• I'm was actually contemplating dropping support for python2, however, since I haven't discovered any major incompatibilities as of right now, I said Hey, why not support both python2 and python3
• If supporting multiple python nanomsg packages becomes a must, then I would most likely go with some abstract module which would wrap the socket for various python nanomsg implementations.

Cheers!