Fix bug when processing blank nodes with reverse properties

See http://lists.w3.org/Archives/Public/public-linked-json/2013Oct/0046.html
Mention context-sensitivity of @type in section 5.4 Specifying the Type

/cc @msporny @gkellogg

This addresses #301
Add Simon Grant to Acknowledgements

This is related to #301
Merge pull request #304 from lanthaler/bnode-reverse-bugfix

Fix bug when processing blank nodes with reverse properties
Make API non-normative
Make reference to WebIDL non-normative

This makes json-ld/json-ld.org#238 moot