Tests for #59 - a manually specified OneToOne should behave
the same as an implicit one.
Update model tests to include our new InheritanceManagerTestChild3
as part of the emulation of select_subclasses.

Test still fails because of #59.
Fixed #59 - manually setting the parent relation
via a OneToOne should present the same behaviour
as the implicit ptr Django generates on
subclasses.

Thanks to Eran Rundstein for reporting the issue
and proposing the fix.
Added test to explicitly demonstrate manually defined related_name
usage works in select_subclasses going forwards.

Also fixed fragility of 2 tests introduced in ae71f9080e3eb83b3d2a98294b28dd3ba711196b
Merge pull request #89 from kezabelle/bugfix/named_onetoone

Allow InheritanceManager to work with manually specified OneToOnes