Maybe add support for automatic bundle-loading from assets.py files,
Django-like. However, since we have no single, global environment, we'd
prefer to collect all "Bundle" instances from a module, rather than
attempting something like g.assets_env.register().

Using loaders is currently somewhat verbose, it'd be more micro-framework-like
if we  could say "assets_env.load('yaml', ...)".

Now that we officially require Flask 0.8, and are no longer testing
older versions, remove the support code for those older versions.
