Display multiple timeseries in one chart

Rework the timeseries class to allow a single timeseries to hold multiple "series" of data (badly named). If a timeseries has more than one series, it displays all series on the same chart.

This also add machinery that will help #42.

@adamlewinter I've pushed this to the dev site (http://gadomski.github.io/glac.io/locations/hubbard/data.html and friends), if you have a moment could you flip through and make sure things look ok? One thing I was wondering is if we should put some sort of indication that a given time series is pulling data from two sensors.