Refactor AdminMailer and add default mailer style

* Rename PaymentMailer to AdminMailer and update naming/namespacing
* Pass args to AdminMailer by ID instead of object
* Rewrote the email, added rewards text, and implemented a default email style
Add notification settings to admin and toggle for admin payment
notification email

* Refactored nav header to accept any value
Merge pull request #112 from Crowdtilt/improved_emails

Refactor admin payment notification email and add toggle for admins to enable/disable