Create API endpoint to trigger reminder emails

This adds a new resource API of `/reminders/email/send`. `POST /reminders/email/send` represents the "act of sending" and the Send resource represents the data associated to this act, such as `startRemindOnDateTime` and `endRemindOnDateTime`.

These are the concepts (I think).
- The resource is Send, but the Reminder and Email resources are related.
- There is one new API - `/reminders/email/send`.
- The API currently has one operation, `POST /reminders/email/send`.

The code added is currently using these conventions, but is still a work in progress.
- Modules
  - `SendModule`
    - it currently has one operation - `POST /reminders/email/send`
- Models
  - `Send` - stores the data associated to this API's act of sending
- Tasks/Sending
  - `PostRemindersEmailSend` - which matches up to the operation of Send
  - `SendEmails` - sub-task for sending emails
- Tests
  - Modules/Sending
    - `RemindersEmailSendTests` - test class responsible for integration testing the `/reminders/email/send` operations (currently only one - POST)
  - Tasks/Sending
    - `PostRemindersEmailSendTests` - tests for `PostRemindersEmailSend` task (exact purpose is TBD)
    - `SendEmailsTest` - tests for `SendEmails` subtask

Todo
- [x] ~~Determine if "Trigger" is the appropriate resource name~~ It wasn't.
- [x] Fix code formatting (need https://github.com/codeforamerica/denver-schedules-api/issues/14 to be completed)
- [x] Need real implementation of [FetchDueReminders](https://github.com/gregoryjscott/denver-schedules-api/blob/email-triggers/Schedules.API/Tasks/Reminders/FetchDueReminders.cs)
- [x] Update documentation with Mandrill environment variables
- [x] Setup Mandrill environment variables for Travis
- [x] Change to sending post data that determines the window of "due" reminders
- [x] Figure out why Travis still doesn't see the environment variables and then revert https://github.com/gregoryjscott/denver-schedules-api/commit/62add82ccf743e847ad9dc4e4a3b18666ffa0a8f
- [x] Flag emailing tests so that they don't run on Travis
- [ ] Clean up commits
Think I'll move Gemfile and Gemfile.lock to a separate PR.