Dashboard page capabilities filter hook

Added filter hook to allow other users (non-admins) to view Dashboard Stats.

Changed 3 instances of `'manage_options'` for the filter hook `a360_dashboard_capability`, so as to allow editors or other roles to view the Dashboard Statistics page.

As [discussed](http://wordpress.org/support/topic/feature-request-add-filter-hook-to-allow-editors-viewing-stats) in wp.org forums.