final group chat changes as well as documentation

- comments have been added to the code
- basic structure setup to update & storage user chat settings
- clicking on a hangouts button now ultimately result in a group chat session being created for that course   with the corresponding hangouts url (the actual insert is tried from an AJAX JSON call made from the Google+ Hangouts app)
- Hangout user chat settings added
- group chat history page updated to account for Google+ Hangouts lack of support for retrieve chat messages
- documentation