adding followers/following and other improvements

[msramalho]

this PR does the following:

• fix test edge cases
• get_followers(self, username=None, user_id=None) to get the given user's followers
• get_following(self, username=None, user_id=None) to get who the given user is following
• load_other_user to load the details from any user even if not the authenticated one
  • had to generalize get_user_url_by_id, get_user_url, get_data_by_user_id to work for users other than the one currently loaded
• get_user_id_from_username which proves useful for some API queries
• tests

I know this brings more than a single change but I invite the authors to ask for updates or edit the PR themselves in a way that makes sense for them and the project.