Fixing broken-ness and adding some error support

- Login wasn't working because I wasn't limiting my conversion from
  input fields -> form data to *just* the login form. The values from
  the 'Create Account' form were overwriting them, so instead of logging
  in, it was just failing to create an account ("username is already
  taken").

- Added an error condition to catch "Can't display that." text response.
  This happens when login has failed for some reason, and a URL that is
  only available when logged in is fetched.

- Added a try-except block looking for IndexErrors when grabbing the
  next link.

- Bumped the version number in the user agent string.

- Added some debugging support to HNSession.login()
Bump version number in setup.py