groups similar methods/classes

[ajoaoff]

Original issue opened by @erickvermot at kytos#315.

Group similarities as in HWAdress and DPID.
The have very similar pack functions for example.
Maybe should they be derived from a same base class ("colon separated hex
values...")
Same with echo request, echo reply, and hello... and many others..

Maybe implement using Mixin classes, as proposed by Diego. I agree;