Add token authentication

This changes `/reminders/email/send` to require an API token. The `/authenticate` endpoint is added to allow clients to receive a token.

Other updates:
- Removes the check that the Remind On date isn't in the past when firing off emails. This makes testing difficult and probably doesn't add much value anyway.
- Refactors `FetchDueReminders` to take reminder type as an input
- Introduces 2 new environment variables
- Introduces Rake to the project and adds a task `rake send` that will send reminder emails
- Adds system test that checks that all environments variables exist. I'm going to say this is good enough to close #63.

Todo
- [x] Replace admin/admin with something real
- [x] Determine if the rake task should just assume current day as the Remind On date
- [x] Solve timeout issue when sending lots of emails
- [x] FetchDueReminders isn't filtering to just email reminders
- [x] Add new environment variables to wiki
- [x] Do something about the hard-coded URLs in Rakefile (for environments other than local)
- [x] Clean up commits