Implemented RegistrarName.authenticate().

Program now authenticates successfully to Name.com and obtains a session
token. Includes tests.

Also renamed "Invalid JSON" errors to "Invalid response". Invalid JSON
should only show up from the _request_json() method, otherwise it's
confusing, since in authenticate() and in get_current_ip() it is valid
JSON just not the data we need.

Successful tests will now have a more real-world response sample instead
of a close assumption like I had before.