Mixpanel tracking

Added a simple mixpanel module that can keep track of user-generated events.
This initial implementation added some tracking for content events.
Mixpanel - Authentication events

Added tracking for
 * Log in
 * Log out
 * Impostering
Mixpanel - Discussions

Tracking the following events:
 * Creating a discussion
 * Updating a discussion
 * Deleting a discussion
 * Sharing a discussion
 * Posting a discussion comment
 * Deleting a discussion comment
Mixpanel - Folders
Mixpanel - Groups
Mixpanel - Storing delta of members so they are summable
Mixpanel - Users
Mixpanel - Search
Mixpanel event tracking updates

 - Tracking which property updated on resource updates
 - Tracking which strategy a user used to log in when he logs out
 - Renamed RETRIEVED_ to GET_
 - Various cleanups