- added clearResources public api method, to flush all resources currently being loaded :
 * this also means a change in the assetManager, as it now returns a deferred (to be cancellable)
instead of a promise (this needs to be refined further)
 * clearResources also accept an options object like {clearCache:true} to specifiy whether or not
the assetManager should also flush its caches (free up memory)
 * a bit rought bit fixes #21 (though it cannot stop already started downloads at this stage)

- added API documentation basics to README, including new method above