Make it possible to use asset_pack without dart:html

The code is now serpeated into three libraries:

* asset_pack_common: Contains most of the code that is independent from dart:html. AssetManager is now abstract.
* asset_pack_browser: Contains loaders that depend on dart:html. Also contains a AssetManagerBrowser that setup the right loaders.
* asset_pack_standalone: Contains loaders that depend on dart:io. Also contains a AssetManagerStandalone that setup the right loaders and a simple text loader.

This solves issue #37.