Added recursive dependency loader for test page

* The page can now load dependencies of the required language recursively
* It first loads the dependencies, then the main language
* It is implemented using promises

The [promises polyfill from jakearchibald](https://github.com/jakearchibald/es6-promise) is used - the polyfill should fallback to browser behaviour, if native promises are supported.
Merge pull request #330 from apfelbox/test-dependency-loader

Added recursive dependency loader for test page