v0.5.2

Changelog:

• Adding OtherUserException, raised when the username has been changed (wiuth set_username()) and then a method has been used which cannot work on another user than the logged in user.
• Adding get_data_by_user_id() #92 which allows getting data more arbitrarily, e.g. shopItems or lingots. And adding get_user_url_by_id() to support that.
• Fixing get_audio_url() for new duolingo changes
• Fixing get_daily_xp_progress() #96 to use system midnight, in case duolingo reports XP progress in the future.
• Ending testing on python 3.4, which is out of support, and adding testing for python 3.9 dev.
• Fixing and expanding tests so that they can run daily in travis, and alert on new Duolingo API changes when they happen.