Include code examples for integration and functional tests in docs.

Fixes #1001
@tseaver @stevepiercy 
The includeme function does not currently have a referable API which can be resolved. 
Currently pyramid.config.__init__ uses string lookup using getattr to obtain a package's 'includeme' function,
but  pyramid itself does not define one.
We could add a module interface and document the function there, and perhaps add tests that a package's include me function implements this interface correctly.  I  think it's probably a separate issue.