Bug fixing in LD parsing

Change @id to id one more time, and deal with errors in turtle parser
Add function to generatate Skolem IRI

See http://www.w3.org/TR/rdf11-concepts/#section-skolemization

Also add attribute to models that will potentially need them.
Use Dexie.js as a persistance layer

This replaces the default Backbone.sync with a wrapper around Dexie.js
(aka IndexedDB). Relationships within models defined using
backbone-relational will automatically be denormalized from objects
being saved. The inverse happens when objects are fetched, however, at
the moment, I've only restored a shell object that looks like
`{ id: id }`. I'm not sure if I want to do the fetching using Dexie or
backbone-relational's fetchRelated method.

Still a ways to go but this is a start.