Clarified tests, fixed BelongsTo.prototype.create

Added clarified test-case based on previous documentation example.

Fixed BelongsTo.prototype.create - although the foreignKey was set on
the model instance,
it was never actually persisted, unless you'd issue a separate call to
save the 'parent' model.
Refactor polymorphic relations, fix inverse #215

See #215 - when creating a related item through a the inverse of a
polymorphic HABTM relation,
the through-model was not created correctly. By refactoring the
specifics into the `polymorphic`
property of a RelationDefinition, it's now possible to handle this
correctly.
Merge branch 'fix/relation-tests' into development
Refactor embedsMany - auto-save parent

With this change, saving an embedded model now correctly updates the
parent model.

Before, a separate `save()` call on the parent was required, contrary
to other relation types.
Implement embedded.destroy() integration
Fixed conflicts
Fix remaining merge issues