Add mocked backend

Summary:
* create a gulp task 'mock-backend' for running a simple node connect server
  * delivers all json files within ./mocked_backend folder to localhost:8081/
    * the json extension will be truncated
      * so 'mocked_backend/test.json' maps to 'localhost:8081/test'
  * cors enabled
* add a posts.json files with some dummy posts
* add command line flag '--mock-backend'
  * via gulp-util's env hash
  * when the flag is used when calling the default gulp task on the command line ('gulp --mock-backend')
    * the mock-backend task will be started (as a dependent task)
* set the backendUrl config variable in app.js (previously hard coded) via envify
  * currently to 'http://ushahidi-backend' or 'http://localhost:8081'
    * based on the state of the mock-backend flag
    * more advanced enviornment configuration scenarios are in general possible
* refactor gulpfile a bit
  * e.g. introduce a few helpers

Test Plan:
* run 'npm install'
  * run 'gulp mock-backend' for running the mocked backend explicitly
    * open 'http://localhost:8081/posts'
  * run 'gulp --mock-backend' for running the default task, with the mock-backend enabled
    * open the app, check that the data delivered by the mocked backend is used now instead of the 'real' backend

Reviewers: shadowhand, rjmackay

Reviewed By: rjmackay

Subscribers: seth, rjmackay

Maniphest Tasks: T824

Differential Revision: https://phabricator.ushahidi.com/D375