Allow 1 second granularity for downsampling

[kevinburke]

There are metrics where I would like to show data points more than once per minute.

[discogestalt]

There is a downsampling choice of "None" which will show you all points as returned from OpenTSDB, with the caveat of course that if you are collecting at sub-1 minute intervals and you try to graph more than an hour (or less, depending on how many metrics you're trying to show on the graph), you're browser will quickly get very unhappy with you. Not to mention that you'll potentially be trying to graph more points on a line than you have display pixels.