feature(instruments): Add functions to check if a metric is being
tracked.

This helps with testing.
feature(instruments-RecordWork): Add a new type for measuring work.

This new type allows you to measure work for a function in a generic
way. By passing a callback on instantiation, you can get a 'mutated'
callback that will stop the work timer when the callback is invoked.
fix(RecordWork): expose functions used purely for testing as part of a testFunction object.
fix(RecordWork): instead of returning the mutated callback from RecordWork.startWork(), return the object itself
Merge pull request #10 from racker/recordwork_obj

RFR: RecordWork instrument type for timing and counting generic functions