Async cover loading from URL, Elapsed time support, etc.

Loading of a cover from URL (as well as loading from a file from now on) is done in a separate thread asynchronously and non-blocking.
If image is absent then default image named "no-image" will be used instead.
Elapsed time attribute introduced, allowing to be used along with duration.
I propose to change the namespace from window.plugins.remoteControls to just window.remoteControls, which means it can be accessed via simply remoteControls as well.
Readme is updated to include these changes.