move gulp sub modules to its own folder and rename gulp-options.js

Summary:
* just moved the mock-backend.js and node-server.js to 'gulp'
* renamed gulp-options.js to '.gulpconfg.js'

Test Plan:
* check that gulp works as usual
  * especially the node-server/mock-backend tasks/options and the config in .gulpconfig.js

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: seth

Differential Revision: https://phabricator.ushahidi.com/D402