Build system with grunt

Build instruction
- npm install
- grunt build

To generate dependencies hashes:
- grunt freeze

Notes:
- Assets loaded from javascript need to be placed in /src/assets
- The build script assumes that all the "pages" are in the root folder of the site. In our case there is only index.html. And especially the script assumes that all html files in subfolders are loaded dynamically from a page in root folder (eg /src/pages/*). If we add other pages, they must be in the root folder (/src/)