[#706] PEP8 Compliance

If you want me to actually remove the changes from `dht.py`, `market.py` and `transport.py` let me know.
I can also add a test for PEP8 compliance.
It's only 2 characters too long. Renaming `signature` to `sig` would also fix
it.

I figured it looked nicer than these: 

    if CryptoPeerListener.validate_signature(signature,
                                             signed_data):

    if CryptoPeerListener.validate_signature(
            signature, signed_data):

Although I guess the 2nd isn't too bad, I try and avoid multi-line ifs as they
can be jarring.

And it kind of splits up the "something is happening" and "some condition is
being tested" abstractions that were previously combined in the `if`.

I think a cleaner solution would be to change `validate_signature` to
`is_signature_valid` or something similar. But I wasn't trying to rename all
your functions on my first PR. Maybe one day, with permission :P

Let me know how you want me to handle it.