First version of OAuth2 signin/signout feature

Summary:
* add simple sign in form and use OAuth2
  * used OAuth2 flow: Resource Owner Credentials
  * use a http interecptor for catching unauthorized http responses
  * add a ng service for the authentication process
* signin/signout button (as a ng directive)
* as a proof of concept: add Posts resource as a 'protected' backend resource
  * add data service for posts
    * uses angular-resource
    * replaces the dummy post data
* introduce BACKEND_URL and API_URL constants
* add protractor for e2e (integration) specs
* add e2e specs for signin/signout

Test Plan:
* run the specs: 'protractor test/protractor-conf.js' (or 'npm run protractor')
* try to open /posts without beeing signed in
* then sign in and try again

Reviewers: seth, spaudanjo, shadowhand

Reviewed By: shadowhand

Subscribers: rjmackay

Differential Revision: https://phabricator.ushahidi.com/D319