Feat/with context

A new `test_with_mock/6` function has been added to the Mock module.
This new function differs from the `test_with_mock/5` function by
excepting a context argument.

This allows `test_with_mock` to be used with the standard ExUnit setup
procedure, enabling information to be shared between callbacks and      
tests.

Additionally an example of the `with_mock` macro has now been added to the   
README.