do not inherit Context::User in Context::Me

otherwise both tabs will be active in the ui. Inheritance was used for the model_name
class method. Now inheriting this from Context and setting the wrapped_base_class on
each inherited class. This also removes duplication between Context::User and
Context::Group
Merge pull request #65 from azul/bugfix/two-active-tabs

do not inherit Context::User in Context::Me