Completed ASCII armor support

I've added the 'enarmor' function including a unit test. 

Currently the public API visible via "dir(OpenPGP)" is polluted with imported standard modules. A user of OpenPGP-Python might think they belong to the "intentinally published API" an uses them, e.g. "OpenPGP.unpack".

This problem could be solved when those module are renamed on import with a leading underscore using the "as"-keyword. This way they would be hidden in the result of "dir(OpenPGP)". So only the real published API remains visible. I think that would clarify usage a lot.