Add an isNotNull to count/determine whether a metric had any values.

In a pool where I have a flexible number of nodes, I need to be able to
determine how many nodes were alive at a particular time. (Well technically
the closest I can get is how many nodes were reporting metrics at a
particular time.)

This is a function that just returns whether a metric is non-null for each
measurement. You can create a 'nodes alive' graph with something like:

sumSeries(isNonNull(server*.cpu-1.cpu-idle))
Merge pull request #365 from burr86/master

add an isNonNull function