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.
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