Adding structure for SMS donation modules

Let me know what you think/if you've got questions on this one @tongxiang 

#### What's this PR do?
This starts development of the internal module for handling SMS donations.

#### Where should the reviewer start?
Entirety of donation functionality is intended to live within the `lib/donations` folder. There are 3 main components, and thus 3 route endpoints, that I see being needed for the initial Donors Choose implementation. Those have been stubbed out in `donations.js` and in the controller. The model is my best guess right now at what data we'll need to track through the user's process of submitting a donation.

#### What are the relevant tickets?
SMS-34, SMS-35, SMS-36