allow user info schema customization

[Nimo-19]

Allow user info schema customization to correctly parse Oauth spec lists. should fix: #88

The idea is to configure a list of values of the style <claim>:<ParamDefinitionName>
During the get_user_info step the configuration is parsed into its corresponding ParamDefinition form oic/oauth2/message.py:

SINGLE_REQUIRED_STRING = ParamDefinition(str, True, None, None, False)
SINGLE_OPTIONAL_STRING = ParamDefinition(str, False, None, None, False)
SINGLE_OPTIONAL_INT = ParamDefinition(int, False, None, None, False)
OPTIONAL_LIST_OF_STRINGS = ParamDefinition([str], False, list_serializer, list_deserializer, False)
REQUIRED_LIST_OF_STRINGS = ParamDefinition([str], True, list_serializer, list_deserializer, False)
OPTIONAL_LIST_OF_SP_SEP_STRINGS = ParamDefinition([str], False, sp_sep_list_serializer, sp_sep_list_deserializer, False)
REQUIRED_LIST_OF_SP_SEP_STRINGS = ParamDefinition([str], True, sp_sep_list_serializer, sp_sep_list_deserializer, False)
SINGLE_OPTIONAL_JSON = ParamDefinition(str, False, json_serializer, json_deserializer, False)

The OpenIDSchema from oic/message.py is now extended and the claim can be parsed correctly by the pyoidc package.

I didn't get a better idea for configuration. JSONField in the plone ui clashed somehow with the acl_users ui.