"Explicit" rules

Hi,

this pull request implements some changes to implement explicit rules.  Before, rules were always created implicitly by the ```is_allowed``` call.  This is suitable for the rate limiting use case that motivated the module in the first case, i.e. limiting specific clients.  I need the module to put a (dynamic) limit on the backend, though, in order to prevent overload situations.

With these changes, it is possible to change the windows definition for a rule after it has been initially created (that would not work before).  Also, a new call ```set_windows``` has been added that can be used to create a rule or re-set the limits for a certain rule.

As changes to such explicit rules are an administrative measure, I have added code to log rule changes for explicit rules.  Also, the module now logs errors arising from invalid window definition strings using syslog.

Thanks for the module, it works great!
Hans