instrumentationPrefix() and instrument() take an option called 'nodejs' that can be used to enable Node.js-specific features
associate invocations with epochs, recording how deep into the epoch's stack each invocation is so that later we can query for all the top invocations in each epoch
automatically generate epochs in Node.js by overriding EventEmitter.prototype.emit
Tracker extracts the bookkeeping for all the track*(), *delta() calls. epochTracker uses it for handling the epoch-related calls.
throw errors instead of strings
fix copy/paste regression in 7af8585 that broke the tests