FIX: Fix first dt

Closes #248.

Ready for review/merge. I was also +1 on @lcampagn's proposal :)

I changed `Timer.start()` to set `_last_emit_time` to the current time, and thus got rid of the conditional in `_timeout`. This should work as long as we're guaranteed that `start()` will be called before `_timeout()` is ever called -- I assume this is how the `Timer` class functions.