Allowing inverse relationships to be nullable.

Now we can prevent `DS.Model.inverseFor` from automatically looking for
a relationship and identifying the wrong relationship in some cases
Merge pull request #1109 from rzurad/null-inverse-relationship

Allowing inverse relationships to be nullable.
Require application.Store to be defined - Fixes #1084
mixup asset file
bump ember - 1.0.0-rc.7