oauth2test.Conversation gets expect_exception

Adds the expect_exception parameter to the Conversation constructor.
Already implemented in base-class and oictest by previous commits.
oauth2test/oauth2_operations.py -> oauth2test/operations.py
Test for presence of state in auth-response

The test includes the state paramter in the authorization request,
ensures that there is a state-parameter in the authorization response
and that the value of the parameter is identical to the value in the
request.
Mark need for interaction better.
Makes inheritance of RequestWrappers easier

When the test-dictionary is changed in __init__ it makes it unclear in
subclasses where the tests are defined (since changes performed in
__init__ can override changes to static variables in subclasses).
Moves identical class to base-module

ErrorResponse-wrapper is identical for both protocols.
Test for a token request with invalid grant type
Whitespace and faulty default values

Removes default-value for Check subclasses (which doesn't work anyway).
Merge pull request #9 from asheidan/feature/tests

Tests