Resource manager

Implemented a remote resource manager (`__anm.resource_manager`), which:

* Allows to subscribe with any number of callbacks to any number of resources and they'll be called when each of these resources was either loaded or failed to load.
* Stores the cache of all requested resources among with waiting and errors queue
* Do not loads any resource with it's own code, just requires a loader function.

Integrated it to `Sheet` class and `audio.js` module.

Also:

* New `RES_LOADING` state for the player
* New `RES_LOAD` event for the player which fires when player started to load resources
* `LOAD` is now fired only when all remote resources (if there were any) were loaded or failed

How to use:

* Get `__anm.resource_manager` instance
* When you need to load a resource, call `.loadOrGet` method of resource manager with:
    * URL of the resource
    * Loader function (it will not be called if resource is already in the cache) which gets `onsuccess` and `onerror` callbacks as arguments and should call them in the corresponding situation (like deferred AJAX call)
    * Callback function which gets the resource value when it was successfully loaded
* To listen for specific resources to complete loading, call `.subscribe` method with array of required URLs and a callback (or array of callbacks) to be called when these resources are loaded or failed to load (or some loaded and some failed)
* If you have a local static resource and some subscriber to it's URL, just call `.trigger` method with URL and it's resource to force resource manager to notify subscribers for this resource. If it failed to load, call `.error` method with URL and error instance