Report a clearer error when a transformer class is given

[tcalmant]

Summary

Giving a transformer class instead of an instance, i.e. load(fd, MyTransformer)
instead of load(fd, MyTransformer()), made the parser call
create_instance() on the class itself. The class description was then given
as the self argument, and the call failed with:

TypeError: create_instance() missing 1 required positional argument: 'classdesc'

which points at the internals of the parser instead of the call of the user.

v2 and v3 now check that argument up front and explain what to do:

TypeError: Transformers must be given as instances, not as classes:
got the class MyTransformer, did you mean MyTransformer() ?

The check only rejects classes: transformers are duck-typed, so an instance
which does not inherit from ObjectTransformer is still accepted, as before.

Test plan

• TestTransformersArgument added to the v2 and v3 test suites: it checks
  that load() and loads() both reject a class and that the message names
  the transformer, and that instances are still accepted, with and without
  inheriting from ObjectTransformer.
• Full test suite green.

Fixes #54

[coveralls]

coverage: 92.256% (+0.02%) from 92.232% — fix-v2-transformer-instance-check into master