CORS and flask-restful

* Implements `flask-restful` extension. Resources are classes with a method named after each supported HTTP method.

* Fixes *CORS* without manual insertion of additional headers. [Thanks!](http://flask.pocoo.org/snippets/56/)

* Implements *DELETE* method for `/indicators` and `/scorecards`. No way to use it from the front-end yet though.

* Closes #4 and #5.