A minimal running App

This is a FYI PR, showing the basic usage of backend API. @2blam @vincentlaucy @hxu

General API query pattern:

   * filter
   * groupby
   * aggregate

See Google doc for more details.

See it live [here](http://project.hupili.net/hk_census_explorer/#/test).

@hxu , we use `$http` directly to test the bridge between frontend and backend. After stablizing your angular service, please help to refactor the query part and ping back. Then we can get an idea how to use it.

TODO:

   * Several TODOs in the code
   * The first plot is very slow. Don't refresh. Just wait until areas are coloured. After that, you can click on the option list and the speed is OK. It's not this slow on my local server. API query is fast. So it looks like FE problem. Please help to identify it.