Feature/configurable storage schemas

The default retention configuration bundled with the module is rather large, resulting in 2.5MB whisper data file for each metric tracked by Graphite. We have a lot of metrics and this would result in way too much data being used for our data files. 

This pull request adds the possibility to configure the default retention policy for tracked metrics. Eventually it would probably be very useful to create a new Puppet resource type to dynamically add new configuration entries for specific rules in storage-schemas.conf, but that's for another effort ;) We'll probably contribute a future improvement if we need to define specific storage rules eventually.

Thanks for the good work, this module works great!