Wire up difference aggregate function

[pauldix]

Takes the difference between one group by interval and another. The user should be able to stack this to specify the behavior like this:

SELECT difference(mean(value)) from key_count
WHERE time > now() - 1h
GROUP BY time(5m)

If no group by time is given, then it should be a transformation function that takes the difference between one point and another. In that case they shouldn't be able to nest function calls like this.

[dgnorton]

Closing. Duplicate of #1825