create MockTestCaseMixin to be used as extension of TestCase class when using mocks
transfer MockMethodCall object to actions
move use-case tests covering more than one class to separate test file
add decode method that checks if correct args are given to mock function
separate tests between unit tests (single classes) and functional tests (use cases)
introduce FunctionInspector class

This class is used to check (via inspection) if given positional and/or keyword
args can be used to invoke underlying function.
FunctionInspector refactored
further refactoring of FunctionInspector
another test for FunctionInspector