Single Mark Helper

[John Riviello]

Since it's currently only possible to have one value for a key (see Issue #54), that results in code like this when you only want to mark a key the first time it happens:

if (!surfnperf.getMark('fetchData')) {
  surfnperf.mark('fetchData');
}

A markOnce helper method to provide that same functionality would be useful.

We should determine a plan around this & #54 before our next release.