Add Conformance section as proposed in #166.

The JSON-LD Data Model is now in a proper section, followed by
a terminology section with general terminology and keywords. The
Conformance section (that refers to the notion of JSON-LD graph
defined in the JSON-LD Data Model section) appears next.

No further changes in this commit.
Merge branch 'master' of https://github.com/json-ld/json-ld.org
Merge branch 'master' of https://github.com/json-ld/json-ld.org
New "stand-alone" Grammar section

Main changes:
- Clarified that JSON terms are directly imported from the JSON spec,
and not re-defined in JSON-LD.
- Moved the definitions of "node definition", "node reference" and
"term" to the grammar. They are JSON-LD constructs.
- Introduced new grammar constructs when needed, in particular
"expanded typed value" and "expanded lanugage-tagged string"
- Re-wrote the grammar in a more consistent way to use the same
vocabulary throughout.
- Fixed grammar when needed (for instance the value in a language
map could be a language-tagged string, which seemed a typo).

Not done yet:
- All normative statements in "Basic Concepts" and "Advanced Concepts"
should be removed (provided they appear in the grammar that is!)
- There remains a couple of places where the grammar is not perfect
(@graph and apparent contradiction in expanded term definition in
particular)
- Examples in the grammar should be moved to previous sections.
- There may be a few places in "Basic Concepts" and "Advanced Concepts"
sections that would benefit from linking to the grammar.
- Some definitions may benefit from a bit of wordsmithing. For instance,
there could be a note around the definition of an IRI that explains
that, whenever the grammar says a value must be an IRI, it is taken
to mean "a string with the lexical form of an IRI".
Dropped normative statement from "Basic" and "Advanced" sections.

Grammar completed as necessary when the normative statements were
not captured yet (e.g. the constraint on circular dependencies
for terms definitions in a context).

Also: IRI definition modified to say that an IRI may either be
represented as an absolute IRI or a relative IRI. "terms" and
"compact IRIs" were also in the list previously, but they generate
IRIs through expansion, and the grammar makes the distinction
when it says that a value can be an IRI, a compact IRI or a term.

Normative statements on JSON-LD processors still appear in the
doc, but should probably be removed.

Also note the "Referencing Contexts from JSON Documents" section
retains its normative wording.

I also updated plural references to terms to include the "s" to
ensure the whole world gets turned into a link.
Markus feedback on recent Conformance/Grammar updates incorporated.

- Created issue #184 to discuss the definition of a JSON-LD
processor. The doc references that issue in the meantime.
- Incorporated Markus feedback to clarify when a JSON object with
a null value is ignored
- Re-introduced statement that discourages the use of empty terms
in "Context" section. I left it in the grammar as well because that
also seems the right place to mention it but, technically speaking,
it could be removed from there since it's not a normative binding.
Merge branch 'master' of https://github.com/json-ld/json-ld.org into gh-pages
Re-introduced sentence to discourage use of terms starting with an "@"

The grammar contains a "SHOULD NOT" statement on that. I re-introduced
a similar statement using "are to be avoided" not to duplicate the
normative statement.
Merge branch 'master' of https://github.com/json-ld/json-ld.org
Removed definition of JSON-LD processors from the Conformance section

Other sections still mention JSON-LD processors, either to describe
how JSON-LD documents are processed or to put constraints on JSON-LD
processors through normative statements. Descriptive text would need
to link to the proper definition of JSON-LD processor in the API doc.
Normative statements that apply to JSON-LD processors need to be
dropped. All of this is being covered by issue #184.
Dropped all references to "JSON-LD processor" from the text.

There remains one occurrence in a note on low-memory footprint
JSON-LD processors, but it is self-explanatory. Also another one
in the Microformat appendix but I'm not clear what the sentence
actually means.
JSON-LD document definition: dropped reference to Linked Data.

"Linked Data" is not normatively defined so shouldn't be referenced
from the JSON-LD document definition in the Conformance section. I
had put the reference only to have some informative description of
what a JSON-LD document is, but that's already expressed in the
introduction anyway.
Merge pull request #194 from tidoust/master

Conformance section and grammar updates