charm.Bundle (et al) should be renamed to Archive

[Dimiter Naydenov]

In the charm package there are several places where we use the term "bundle" with the meaning "a zipped charm archive". There are also several related names in the state package. In all of these cases "bundle" should be renamed to "archive" (e.g. charm.Bundle -> charm.Archive; state.Charm.BundleURL -> state.Charm.ArchiveURL, etc.). This renaming is needed because we started using the term "bundle" inside juju documentation, GUI (e.g. officially) to mean "a collection of related charms, usually exported from a running environment".

In the API packages we are already using Archive instead of Bundle (in method names, comments, etc.), but the changes in state will lead to schema changes, so we need to consider them carefully.