Add support for the integral function

The integral function shows the sum over time, similar to a continuous
addition function. Useful for finding totals or trends in metrics that
are collected per minute.

Example: integral(successes.perMinute)

This would start at zero on the left side of the graph, adding the successes
each minute, and show the total successes for the time period selected at
the right side.
Add support for derivative function

Port the graphite derivative function. It calculates the delta
between consecutive data points of a running total metric.

(Note: doesn't normalise for time, as a true derivative would.)