internal/v4: resolve ids
fixes after rebase
internal/chamstore: additional tests
Merge branch 'v4' of https://github.com/juju/charmstore into 012-resolve-id-2

Conflicts:
	internal/charmstore/store_test.go
	internal/v4/api.go
	internal/v4/api_test.go
Merge pull request #29 from rogpeppe/012-resolve-id-2

internal/v4: resolve ids

This allows all charm store paths to contain charm URLs that are
not fully specified.