Adds breach-mitigation-rails gem to protect app.
Adds base mailer for Sales.

* Adds text version of Sale invoice.
* Adds send_by_email action to Api::SalesController.
Adds send_by_email function to Entities.Sale

* Wires functionality to call Api::SalesController#send_by_email from
  an Entities.Sale object.
Email button on SalesApp.Show view should appear if customer has email.
Adds behavior to send Invoices through email.
Adds sample config file to configure ActionMailer
Organization of mailers into one class TransactionMailer

* Mailer has two methods: invoice and receipt that relate to Sale and
  Payment.
* Wires routes for emailing payment receipts.
* Adds payment receipt email view.
Adds ability to email Payment receipts through the backbone app.
Merge branch 'develop'