valuehistogram: new simple, value-based histogram

This change adds a new valuehistogram panel to support value-based
histograms.  Unlike the previous histogram panel which plots the data
using fixed time-interval buckets, this type of histogram buckets the
data according to the values themselves.  This type of graph provides
a high-level perspective of a field's value across the entire time
interval.

This is simply a more recent rebase of https://github.com/elasticsearch/kibana/pull/622 after a long hiatus in development.