charm: add comments to Bundle type
Merge pull request #13 from rogpeppe/004-bundledir

charm: add support for reading bundle directories
charm: factor zipOpener from CharmArchive

	This is preparatory work for adding BundleArchive
charm: make zipOpener an interface
Merge branch '005-bundlearchive-1' into 006-bundlearchive-2

Conflicts:
	charmarchive.go
rest of it