Advertise the Non-RDF Content as a LDP#NonRDFSource.

http://www.w3.org/TR/ldp/#h5_ldpnr-type
> 4.4.1.2 LDP servers exposing an LDP Non-RDF Source may advertise this by
> exposing a HTTP Link header with a target URI of
> http://www.w3.org/ns/ldp#NonRDFSource, and a link relation type of type
> (that is, rel='type') in responses to requests made to the LDP-NR's HTTP
> Request-URI [RFC5988].

Fixes https://www.pivotaltracker.com/story/show/78046460
RdfAdder and RdfRemover should sanity-check against adding existing mixins or trying to remove the primary type of a node
Update to use new preference PreferMinimalContainer instead of deprecated PreferEmptyContainer
Update Preference-Applied header to match RFC 7240

Fixes https://www.pivotaltracker.com/story/show/78108258
Merge pull request #451 from fcrepo4/ldp-conformance

Updates to conform to the LDP API