GET /activities is now a promise
Cleaned up /activities route:
  added activityHelpers.js, moved some repeated code there
  GET and POST now use promises from the Q library
Changed error handling for /activities POST
/activities/activity_id now uses promises
/activities/activity_id PUT now uses promises
activities/:activity_id GET now uses promises
Added multer instead of busboy, cleaned up image handling code,
added more promises in lieu of callbacks
/activities/:activity_id/images POST now uses promises
activities/:activity_id/images/:image_number PUT now uses promises
/activities/:activity_id/images/:image_number GET now uses promises
Removed leftover middleware from multer form handling
Added comments to activity routes
Merge pull request #8 from gjtrowbridge/promises-instead-of-callbacks

Promises instead of callbacks