Treat reverse term definitions more like regular definitions

This addresses #253
Clarify relationship of JSON-LD and RDF

according the resolutions made in the 2013-06-11 JSON-LD telecon. See http://json-ld.org/minutes/2013-06-11

This addresses #222
Implemented resolutions to address some of David Booth's LC2 feedback.

The resolutions can be seen here: http://json-ld.org/minutes/2013-06-11/
Removed periods at end of phrases in a bulleted list.
Minor wording change based on feedback from Markus.
Integrating changes from Dave Longley related to "Usable as RDF" section.
Fix reference to RDF Concepts
Add statement saying that JSON-LD's data model is closely related to RDF's

as suggested by Pat Hayes.