Updated JSON-LD grammar

I updated the syntax spec [1] to formalize the grammar addressing issue 114 [2]. I believe the grammar is consistent with the rest of the document, but it probably needs updating, along with other parts, for newer resolutions. It could certainly use a review.

I'm also using some new ReSpec features including example titles, issue numbering (links coming with a future ReSpec release) and automatic section reference text.
Gregg

[1] http://json-ld.org/spec/latest/json-ld-syntax/
[2] https://github.com/json-ld/json-ld.org/issues/114