Ref #195 : Add support for the traverse argument

Allows to provide a traverse argument to our Service, that will be used
by cornice for route declaration :

foo_service = cornice.Service(
        name="foo",
        path="/foo/{id}",
        traverse="/bar/baz/{id}")
Ref #195 : Add some docs

Here we document the traverse argument :
* In the Service class' docstring
* In the docs about routing
Merge pull request #196 from tonthon/master

Ref #195 : Add support for the traverse argument