Use plain strings for response statuses.
Replace identity_url attribute with a method reading the endpoint.
Removed Response.getDisplayIdentifier

For now, settle with response.endpoint.claimed_id. It'll probably
go away completely in the future.
Shorter SuccessResponss.__init__
Inline GenericConsumer.begin into Consumer.begin
Extract OpenID argument name constants from GenericConsumer
Extrat session class lookup to the global level of consumer.py.
Don't attach discover function to the consumer class.
Removed useless test from TestDiscoverAndVerify
Inlined failure assertion testing in TestDiscoverAndVerify
Use mocks in TestDiscoverAndVerify
Don't return a separate claimed id along services from discover(). Apparently, it's never used anywhere.
Move GenericConsumer.complete into Consumer.

Couldn't just inline it, as tests call it directly in miriad
different ways. For now test_consumer is a complete mess of
mocks, patches and arguments that are never used. Probably
should first fix tests.
Converted consumer.complete tests to test consumer.complete

- Use consumer.complete instead of the internal _get_response
- Use standrad mocks and asserts
- Don't depend on TestIdRes base class