We can now read a last-publication-date timestamp from maven-metadata.xml.

A few notes:

* For some reason Aether doesn't deem it worthy to put useful information
  from maven-metatdata.xml into their Metadata object by default, which
  means we have to parse the stupid thing.   We should see if we can
  hook our own parser for metadata which automatically pulls out
  the properties we need for publication date.
* ChainResolver is NOT working in the spec tests with the new Maven resolver.
  We must be returning failure in a way which does fail, but breaks the chain.
* Our test for -SNAPSHOT passes if you flip the order of resolvers, except
  the above issue with ChainResolvers means it's not passing generally.
* We need some new mechanism to grab a publicationDate in the event that
  maven-metadata.xml is stale.
* If we have all the maven-caches in the SAME location, it could lead to -SNAPSHOT hell
  across resolvers.