[WIP] Contexts

[TShapinsky]

Gabe's synthesis from yesterday's developer meeting:

• api calls that take shape collections should take a context as an argument
• default to the context for a model for all methods on Model
• context is defined programmatically by transitive closure of owl:imports statements
• a model's manifest contains the owl:imports statements for all graphs/etc necessary for definitions and validation
• each shape collection has a unique URI defined by the "a owl:Ontology" statement"