Convert "partial" lists from RDF to JSON-LD

This addresses #277.
Update the JsonLdContext type defintion in idltest

This addresses #245.
Support documents with a +json media type

This addresses #261.
Clarify that a response MUST NOT contain more than one context HTTP Link Header
Raise an error if multiple context HTTP Link Headers are detected

This addresses #264.
Use the LoadDocumentCallback (previously LoadContextCallback) to load documents

... previously it was only used to load remote contexts.

This addresses #264.
Update toRDF tests to exclude triples with blank node predicates

This addresses #222.
API edits to avoid implying that JSON-LD is not RDF

As promised, attached are suggested edits to the "JSON-LD 1.0 Processing
Algorithms and API" document at

  http://json-ld.org/spec/latest/json-ld-api/index.html

The purpose of these edits is to avoid implying that JSON-LD is not RDF.

The main strategy was to change phrases like "transforms JSON-LD to
RDF" and "converts RDF to JSON-LD" to phrases like "deserializes JSON-LD
to RDF" and "serializes RDF to JSON-LD".

Note that some of the section titles changed as a result, and two HTML
anchors were also changed:

  convert-to-rdf-algorithm --> deserialize-to-rdf-algorithm

  convert-from-rdf-algorithm --> serialize-from-rdf-algorithm

Therefore other documents that reference this document may need to
be updated correspondingly.
Fix markup

No other changes
Syntax doc edits to avoid implying that JSON-LD is not RDF

Attached are the corresponding changes that I see that are needed in the
syntax document: http://json-ld.org/spec/latest/json-ld/

Other editorial issues that I noticed but I did not fix in the attached
version:

1. The Abstract begins: "JSON is a useful data serialization and
messaging format".  Delete "useful" or perhaps delete the whole
sentence.  (Other data serialization and messaging formats are *not*
useful?)

2. The Features at Risk section needs to be updated regarding blank
nodes as predicates.
Minor tweaks to make it clearer what the algorithms are supposed to do
Minor tweaks

Taking into consideration the discussion in https://github.com/json-ld/json-ld.org/pull/287.
Fix typo
Add link to test suite in developer section (renamed from implemenations), and re-style test suite page to match site style.
Change test suite home link to go back to home.
Fix test-suite index page by using properly hosted handlebars.js

The one hosted on GitHub caused problems with Chrome and IE because it is served as text/plain.
Minor tweaks and fixes to the test-suite index page
Rename JSON-LD Play to JSON-LD Playground and create proper page titles

Changed Google Analytics snipping to an account we all have access to.
Remove logo contest page
Reduce and fix example indentation.