Add tests to sample app

[colinloretz]

• Add tests for GET and POST routes
• Adds chai and sinon
• Minor refactor to exports to make stubbing easier
• Remove console.logs that muddied up npm test output

$ npm test

  Server
    GET /
      ✔ should return a greeting message with the Discord application ID
    POST /
      ✔ should handle a PING interaction
      ✔ should handle an AWW command interaction
      ✔ should handle an invite command interaction
      ✔ should handle an unknown command interaction
    All other routes
      ✔ should return a "Not Found" response

  6 passing (12ms)

[JustinBeckwith]

Looks great!