Add timeit() context manager that allows multiple timers to be run simultaneously.
Add test for timer.timeit().
Add a warning deprecating Timer.start().
Use Timer.timeit() in random metrics generator.
Replace direct timing on Timer with EventTimer class.
Docuemnt EventTimer.start and .stop and add tests. Remove deprecation warnings from Timer.__exit__ and Timer.stop since they just add noise.
Add check for timer that hasn't been started.
Remove clumsy additional 'directly' from Timer class docstring.
Fix bug in example in Timer class docstring.
Make EventTimer non-reusable.
Add support for auto-starting EventTimer's created by timeit.
Make deprecation warnings actually DeprecationWarnings.
Merge branch 'feature/issue-667-overlapping-timer-metric-helper' into develop