* Docs for Container::Metadata
* KeyPrefixer extracted
* Renamed MetadataMarshaller
* Specs for existing behavior
Attempt to save the user from themselves to a degree:
* Assume that the user may prepend X-Container-Meta- when performing a
  container metadata deletion
* Similarly assume they may prepend X-Remove-Container-Meta- when
  performing a metata create/add
* Already add the necessary prefix on their behalf during
  create/add/delete if they didn't do it themselves. Less coding for
  them ideally.x