store: Don't allow id in create requests

This fixes #82.
Update CHANGES to detail changes in 0.11.2
First attempt at using Open Annotation Data Model

Annotations can now be serialised into JSON-LD formatted RDF, following
the data model spec: http://www.openannotation.org/spec/core/
Use OrderedDict for OA representation

Because JSON-LD spec recommends keeping @context at the top.
Better checks for absent fields in creating jsonld
Fall back to dict if OrderedDict unavailable
Code reordering and renaming
Small edits&fixes
Use OA context from w3.org