Move dc:describable into the fedora namespace
Add support for ServerManaged preference
Apply server-managed preferences to getResourceTriples()
Stop using PropertiesRdfContext(javax.jcr.Node) constructor
Split out discrete RDF Contexts from PropertiesRdfContext
Mark MODE, MIX, JCR_NT, and LDP as managed namespaces
Allow clients to send Prefer header to control what properties they intend to operate against

This allows clients who only have a partial representation of a resource to use
PUT operations to replace only the triples they are aware of, or provide hints
to the server about what triples they intend to operate on (for e.g. PATCH)

e.g.:

PUT /a
Prefer: handling=lenient;
omit="http://fedora.info/definitions/v4/repository#ServerManaged"

... only non-managed server properties ...

or

PUT /a
Prefer: handling=lenient; received="minimal"

... only the minimal representation ...
Add tests for PreferTag and LdpPreferTag
Merge pull request #549 from fcrepo4/prefer-server-managed

Add a Preference for ServerManaged (and allow clients to exclude Server Managed properties on update)
Add Tombstone resource when a resource is deleted, preventing the re-use of URIs until the tombstone is also removed
Merge pull request #548 from fcrepo4/tombstones

Add Tombstone resource when a resource is deleted, preventing the re-use of URIs until the tombstone is also removed

Resolves: https://www.pivotaltracker.com/story/show/80853212
Merge pull request #560 from fcrepo4/resource-converter

Merge PR #548 and #549