Do not add the c2cgeoform views by default

This PR brings the following:

* It adds a `add_c2cgeoform_views` config directive. By default, when just `config.include('c2cgeoform')` is used the c2cgeoform views and routes are not added. To add the c2cgeoform views and routes the `add_c2cgeoform_views` function should be called on the config object: `config.add_c2cgeoform_views()`.

* It removes the `view_config` and `notfound_view_config` decorators to use the `add_view` and `add_notfound_view` config functions instead. With this the framework does not need to use `config.scan`. Using `config.scan` doesn't make too much sense for a framework, it makes more sense to use at the application level.

Fixes #62.

Note that this PR is based on #60, which should be merged first.