implement meta/revision-info
Use baseurl to search and sort numeric

Use baseurl and remove entries which have different series.
Use numeric sort.
add the revision-info to metaEndpoints

So that it gets tested with all the rest of the meta endpoint machinery.
Document comment the backward Less impl
add decoy charms to revision-info test

Decoy charms prove that they are not found.
rename and unexport EntitiesByURLDesc
s/baseURL.String()/&baseURL/
use RevisionInfoResponse
merge parent
simplify test by using DeepEquals
Merge pull request #77 from jrwren/meta-revision-info

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.