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.
Perhaps search on BaseURL here, then programmatically remove
any entries that have a different series. That way we're not dependent
on the specific string form of a charm URL.

    baseURL := *id
    baseURL.Revision = -1
    baseURL.Series = ""
    h.store.DB.Entities().Find(bson.D{{"baseurl", baseURL}})

Also, is the Sort correct? I suspect it will return results in
lexical order, not numeric, so revision 10 would be ordered
before revision 2. Perhaps better to implement sort.Interface
on []Entity, e.g.

  type entitiesById []mongodoc.Entity
  func (e entitiesById) Less(i, j int) bool {
      return e[i].URL.Revision > e[j].URL.Revision
   }
   etc