use high resolution time instead of Date.getTime

- process.hrtime() returns nanoseconds, and is intended for performance measurement
- also, rename Profiler.sinceLastStep to Profiler.lastStep, since it's just a placeholder for the time of the latest step.
store steps so they can be retrieved later
add flag to disable printing to console
implement getSteps, to return deltas between steps
add precision flag
update readme
update README
timeSinceBeginning and timeSinceLastStep now return desired precision

and ms by default, for backwards compatibility
Merge pull request #2 from capablemonkey/master

Thanks, pretty useful.