[pycrypto] import/export for DSA keys

[Legrandin]

> I've gotten this import and export functionality working using a combination of pycrypto, python-potr, and pyparsing.
> https://github.com/guardianproject/otrfileconverter
>
> Would it make sense to cook this into pycrypto's DSA objects?
>
> .hc

I felt the need for that a few times in the past, and adding it to
pycrypto was high on my to-do list, so I totally vote for that.
It should be a standard format though (like PKCS8), otherwise there is
no benefit over pickle.
I don't think it should require extra dependencies either.