Allow InheritanceManager to work with manually specified OneToOnes

ae71f90 has tests which demonstrate the failure to accommodate the manual name [interestingly, the tests fail different on < 1.6 and 1.6+, presumably as a part of the tickets that allowed grandchildren discovery to work] as documented in #59.
262455a fixes tests which fail with the introduction of the new model under test - the tests are a little fragile in that respect :| [and both are my fault]
a270eef is the actual fix proposed by @eranrund in #59.
dd469a0 implements a test for requesting a subclass by string representation, when that related name is defined manually.