Added a new meta-finder

ActiveBundleFinder finds only bundles that have already been loaded into
the application. It can be used, for example, to send events (using the
Observers pattern) to loaded extensions without loading new ones.
Fixed bug in pushups.finders.meta.ChainedFinder

Omitted import caused NameError to be raised
Fixed bug in EggFinder

NameError when trying to iterate over the environment
Fixed bug in ActiveBundleFinder meta-finder

Was yielding (bundle, loader) tuples instead of bundles