DonorsChoose.org API integration for 3M / Science Sleuth choose-your-own-adventure game. 

#### What's this PR do?
This provides integration with the DonorsChoose donate transactions API. ([Documentation with DonorsChoose available here](http://data.donorschoose.org/docs/transactions/project-donations/)). When a user finishes the 3M science sleuth main game flow, they're directed via a keyword to the donations flow. They enter their 1) **zip code location**, their 2) **first name**, and their 3) **email address** (optional), and our app finds a high-priority (high poverty, greatest need) proposal nearest that zip code, and donates to that proposal. 

#### Where should the reviewer start?
Check out the API integration logic at **app/lib/donations/controllers/DonorsChooseDonationController.js**, then check out the new opt in path config file created at **app/lib/donations/config/donorschoose.js**. 

#### How should this be manually tested?
This should be manually tested using Postman, which can simulate mobile phone requests from Mobile Commons. Contact Tong for a set of Postman configs. 

#### What are the relevant tickets?
JIRA SMS-71