Add `Mocking Service` button to menubar

  - add new menu item, `Mocking Service` button, to menu bar
  - use one of CSS3 checkbox styles to make button look really nice and
    modern; sizing and positioning are glitching from time to time though
  - add `mockingService` AngularJS service to work with remote mocking
    service
  - add tests for `mockingService` service
  - add `mockingService` controller
  - add tests for `mockingService` controller
  - fix `saveMeta` so that it resolves to passed meta, not file
  - cleanup `Gruntfile.js` and add support for CORS proxy as `/proxy/`
    when running `grunt server` for development