implement meta/revision-info

This is very close to the same as ExtendId which was just implemented.

The revision-info path returns information about other available revisions of the charm id that the charmstore knows about. It will include both older and newer revisions. The fully qualified ids of those charms will be returned in an ordered list from newest to oldest revision. Note that the current revision will be included in the list as it is also an available revision.
This doesn't look quite right with respect to the API spec, which
states that the call returns a struct of this form:

     type RevisionInfoResponse struct {
         Revisions []*charm.Reference
     }