Added setMock method

Just a straightforward method to be able to provide an existing mock instead of having to create one inline. Useful for cases where the mocks are shared across tests.

*  Added setMock method in MockerContainer
  * Moved the mocking of the service to a protected method so the exception throwing can be shared
* Added test to show setMock working as expected
* Updated readme to show using `setMock()`