Make API non-normative

These are the proposed changes to make JSON-LD's API non-normative. I think they are quite straightforward:

 - Mark section 11 The Application Programming Interface as non-normative
 - Replace all RFC2119 keywords in that section with statements (e.g. MUST
be -> is)
 - Remove JSON-LD API Implementation conformance class
 - Minor rewording of the abstract and the intro (could be further improved)
 - Change Promise reference

I would appreciate if some of you could have a look just to make sure I
didn't forget anything important.


Thanks,
Markus