Generalize legends and titles

This branch generalizes the title and legend methods from StatmonsterRRD to work for all data sources. Titles will be the common prefix amongst all sources in the graph; the legend will be the unique suffixes in the sources. Optionally these paths can be 'cleaned' by splitting on a char, like StatmonsterRRD does with '.'. This character can be defined per-datasource (or turned off entirely).

I need this for my upcoming aggregating data source.