Also:
- changed http to https in the code
- changed header underlines in the documentation to match the header length

Adds the possibility to create a new key using a custom exponent. Mostly
for compatibility. Also removed the unused parameter nbits from
calculate_keys(). I added a new function calculate_keys_custom_exponent()
so that people still passing a value to nbits don't accidentally use
it as the exponent.

The u'' string prefix was reintroduced in Python 3.3, and since we've
dropped py3.2 support we can avoid the whole py{2k,3k}constants.py mess.

Pickling is now possible, with the added note that one should never
unpickle from an untrusted or unauthenticated source.

This prevents side-channel (such as timing) attacks, see:
https://en.wikipedia.org/wiki/Blinding_%28cryptography%29

Mostly focused on docstrings (''' → """), indentation, empty lines,
and superfluous parenthesis.