Adding platform super property to Mixpanel.
Fixing a typo of a constant related to mixpanel stats.
Fixed formatting issue in WPMobileStats
Changing a few Mixpanel events on the post settings to be properties.

In order to be more consistent, I changed some of the post settings
related events to be properties on Mixpanel rather than events. This
change is done to be more consistent with the Android Mixpanel
implementation. The events I changed to properties were:
"Settings - Clicked Set Featured Image"
"Settings - Clicked Remove Featured Image"
"Settings - Clicked Add Location"
"Settings - Clicked Update Location"
"Settings - Clicked Remove Location"
Changed tracking of session_count for Mixpanel to use super properties

Mixpanel super properties will store values for a user over a period
of time so we don’t need utilize NSUserDefaults to track a
session_count.
Merge pull request #691 from wordpress-mobile/issues/683-mixpanel-changes

An assortment of Mixpanel related changes to get ready for the Android app using it as well