Added anomaly detection support to the stats graph filter, some doc

tweaks / fixes.
The first parameter is the payload name (the graph to apply the detection to). In this case whatever you are setting 'title' to. This allows you to specify anomaly detection of any number of cbuf data structures.  I will update the docs with this comment and add some information about when the different algorithms should be used.

See: https://github.com/mozilla-services/puppet-config/blob/master/shared/modules/shared/templates/hekad/sync.toml.erb for more advanced use cases.