Add extended stats to postgres collector

I refactored the postgres collector in a way that *should* be backwards compatible with the previous implementation, and added the option to collect extended statistics at a more granular level. This has the potential to add thousands of extra data points to graphite (which we've found useful but may not be desirable in every scenario), so I've made the extra data optional for now.

The additional data includes table- and index-level stats, as well as the default queries included with Munin for their postgres collector.