Added tracking for multiple people properties per mixpanel 'event'.
Added metrics on last time users performed certain actions.

Added in Mixpanel People Properties to track:
- Last Time Accessed Reader
- Last Time Accessed Theme Browser
- Last Time Liked Article
- Last Time Reblogged Article
- Last Time Loaded Freshly Pressed
- Last Time Commented on an Article
- Last Time Stats Accessed
- Last Time Published Post
- Last Time Notifications Accessed
- Last Time Shared item
Adding in Mixpanel "$created" property.
Renaming method in WPAnalyticsTrackerMixpanel to be more obj-cish.
Renaming another method to be more objc-ish.
Merge pull request #1586 from wordpress-mobile/issue/add-date-people-properties

Add in people properties to track last time users performed key actions