Add a module example on how to use the configuration and the state API

[avpaderno]

The configuration and the state API are new for people coming from Drupal 7. An example module (or two) that shows how and when to use those APIs would help them.

[avpaderno]

Maybe the example module should also show that configuration values for a module that implements hook_config_info() are automatically removed.

This must be postponed on #45 and #43. Before adding an example module for configuration and state APIs, we should at least be the first ones to correctly use them. 😉