collapse exposed introspection to a single object

Instead of creating a lot of helper methods and/or constants, just
create a single method named #polymorpheus, which exposes aspects
of the interface that are relevant for introspection.
add POLYMORPHEUS_ASSOCIATIONS const to interface
add `has_many_as_polymorphs` declaration option
ENV var to specify Rails version for specs
update README to describe new changes & fix errors
version bump to 2.0.0

Since the recent interface changes break the previous API, this
is a major version bump.