Email designs

This is my work on getting the email designs up and running. There is still a lot of work to be done. No merges until all checkboxes are, well, checked off.

Default email designs to be made:
- [x] Test email in backend
- [x] Test email in core
- [ ] Shipment email in core
- [ ] Cancel email in core
- [ ] Confirm email in core

Other:
- [ ] Remove premailer as a dependency
- [ ] Set up tests for the multipart emails.

Added from PR's Discussion:
- [x] Create a layout for the emails
- [ ] Refactor layouts without partials and with controllers (read: clean up mess)
- [ ] Rake task to send emails