Add an example of how to use unit testing for pyramid_mako

This adds a simple example that shows how to use config.include() in a
setUp() function for a test to set up the renderers so that they can be
used inside of views that are to be tested.
Add example assert
Merge pull request #5 from bertjwregeer/docs/unit_testing

Add an example of how to use unit testing for pyramid_mako