Add metaCharmConfig.  Checkpoint.
v4: add charm-metadata endpoint
Update dependencies.
Fixed tests to work.
Merged Roger's work.
Added more API tests.
Merge pull request #22 from bac/api-charm-config

API charm config
Merge remote-tracking branch 'origin/v4' into 010-assertjson-lossless

Conflicts:
	internal/storetesting/http.go