Merge pull request #42 from nemesisdesign/master

filter statistics by AP mac (called station id)
Merge pull request #43 from nemesisdesign/master

find_unaware_radius_accountings query optimized
Merge pull request #44 from nemesisdesign/master

AddUniqueKeyToUsername migration remove_index fix
Added "gestpay_invoicing_enabled" config key
Admins can disable invoicing for credit card verifications that involve payments.

Summary of changes:
 * model changes
 * unit test
 * fixture (for new installations)
 * data-migration (for existing installations)