Shouldn't try to coerce bytes to a string. Instead, print the repr
value (e.g. b'mybytestring').

When running tests with the Python `-b` option, fixes warnings of the
form:

.../python-rsa/tests/test_strings.py:34: BytesWarning: str() on a bytes instance
  print("\tMessage:   %s" % message)
.../python-rsa/tests/test_strings.py:37: BytesWarning: str() on a bytes instance
  print("\tEncrypted: %s" % encrypted)
.../python-rsa/tests/test_strings.py:40: BytesWarning: str() on a bytes instance
  print("\tDecrypted: %s" % decrypted)

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

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.

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