Offer a higher level interface to hello messages

[erickvermot]

It would be very convenient to have a higher level interface to deal with hello messages and the version bitmaps.
The following branch proposes a simple interface where one is able to create a hello message passing the xid and a python list of supported versions:
https://github.com/erickvermot/python-openflow/tree/high_level_hello

this still needs fixing of #378, which is related to #303. And once #379 is closed, the same development proposed here can also be applied to v0x01.

Furthermore, following the same logic as presented here for the creation of the message, the versionbitmap when unpacked could also return a pythons list of versions in order to make its use simpler.

This proposal is intended to facilitate the use of the lib.
This goes with the same idea as https://github.com/kytos/kytos/blob/master/docs/blueprints/EP009.rst (kytos/kytos#306), and also @renanrodrigo https://github.com/kytos/kytos/blob/master/docs/blueprints/EP011.rst (kytos/kytos#311).

[diraol]

See #379 (comment)