More plug-in oriented would be nice. I think copas and ev could both go into their own module, say 'busted.async.ev' and 'busted.async.copas' for example. Then the api for those can be copied by anyone wishing to make an async module for his own scheduler.

Then `setloop(loop)` would either require the module (when `loop` is a string) or use the provided set (when `loop` is a module table)