.get() should allow myforeignkey_id lookups

[David Cramer]

To allow easy accessibility as you can with .create on foreignkey_id calls, .get should allow the same.

e.g.

MyModel.objects.create(author_id=1) # Works
MyModel.objects.create(author=Author(id=1)) # Works
MyModel.objects.get(author_id=1) # Doesn't Work
MyModel.objects.get(author=Author(id=1)) # Works
MyModel.objects.get_or_create(author_id=1) # Doesn't Work