Add README.md updates for polymorphism.
Add test for polymorphic associations.
Add support for polymorphic relationships using the POLYMORPHIC setting.

This commit adds support for two settings:

FOREIGNTYPE - A field that is used to store the type of the owner.

POLYMORPHIC - A shortcut to set FOREIGNKEY and FOREIGNTYPE to the same
value suffixed by "Id" and "Type" respectively.

The type is stored as the table name, which I thought might be useful
for other queries.

The biggest gotcha of this commit is that I flipped the definition of
has_one and belongs_to. gorm is very flexible such that it didn't
really care if it was a has_one or belongs_to, and can pretty much
determine it at runtime. For the sake of the error, I had to define
one of them as belongs_to, and I chose the one with the fields as
the belongs_to, like ActiveRecord. The error could probably be
genericized to "gorm cannot determine type", but I think it's nicer
to tell people DONT DO PATTERN XYZ CAUSE IT WONT WORK. Functionally,
it doesn't matter.
Merge pull request #303 from jnfeinstein/dev_poly

Support polymorphic has-one and has-many associations