Post a timer without overwrite

[jayeheffernan]

This adds support to the .post() method to only post the timer if it doesn't already exist. This is useful for implementing application logic where the timer must fire after delay milliseconds, or sooner if the timer was already set (the current behaviour is to always overwrite and extend the timer). I've added a new parameter, but maintained backwards compatibility.

I'm using this in my app because I needed it. I don't have any tests, but thought you might like to consider it anyway.

[coveralls]

Coverage decreased (-0.5%) to 99.522% when pulling 9beb61c799532a77d213ecaac8b7cc1546d148e7 on jayeheffernan:not_exists into d37a33edaab4235f3ee1f84e0e5c53e68f9130ea on enobufs:master.