Fix parent context

context.parentContext just needs to be a reference to the parent context.

createChildResolver function not required. No need to go to the parent context to do anything as the parent has no need to be aware of the child's existence (it's the child that must reference the parent).

Original createChildResolver function was calling "child" a resolver based on the parent context as this._context is the parent context when the function is called.
Merge branch 'patch-1' of github.com:mmikeyy/backbone.geppetto into fix_54
Fix tests for #54 and add missing ones for full coverage
Merge pull request #61 from creynders/fix_54

Fix tests for #54 and add missing ones for full coverage