Make granularity of graph selectable

[auxesis]

Sometimes on especially large dataset you want the data to be averaged out.

RRD has a resolution function that can do this automagically.

This should be exposed through the UI.

[jessereynolds]

With the addition of the 95th Percentile feature, the visage server now does downsampling of data before sending to the user (currently set to a target of between 1,000 and 2,000 samples per series). This range could be twiddled with a slider in the UI. Similarly, zooming in on the graph could refetch higher resolution samples for the reduced timespan.