Add asynchronous loading mode

[benpious]

It should be possible to load drawables off the main thread. The parser should already be thread safe, so the interesting part of the task here is deciding how to handle the concurrency: whether it should be hidden from the user, whether there should be a placeholder or whether it should be converted to blocking when it's time to display a drawable onscreen, etc.

This shouldn't change the existing priorities for performance, we should still be aiming to be faster than competing libraries even when we're exclusively on the main thread.