Meter: add ref/unref methods

Make it possible to ref and unref the timer that backs the meter.

Without an unref method, a running meter will keep the event loop alive
even when everything else has quit.  That of course is not always the
desired behavior.
test: remove meter.start monkey-patch

Meter#start() is an empty method as of commit 2dc9ce5, there is no
longer any need to monkey-patch it out.
Merge pull request #9 from bnoordhuis/ref-unref

Add ref/unref methods