Allow for caching multiple users' stats

Currently, the way this module is designed and written, it will only display the email statistics for one user. When it is time to clear the cache, the first user to trigger the statistics check will have their statistics stored in the cache.

In order for this module to be more useful to more people, it should allow for caching multiple users' mail statistics.

This needs to be fixed in both 6.x and 7.x.