On execution, reset timer _before_ firing callback

[movitto]

Prevents an edge case where a context switch between firing of the callback and timer reset caused nested timers (timer that is launched within another timer callback) to deadlock.

See issue #9

[coveralls]

Coverage remained the same at 94.465% when pulling 47028246729d07734d392f5ccbd3d275d1efed2a on movitto:nested_timers_deadlock into d9bc2b9ef8e0d189a6eaa60bd0360319204a3dad on chadrem:master.

[chadrem]

Thanks for the PR. When I get some time I'll review the code to make sure this doesn't introduce other problems. Also, I think it makes sense to add the demo you made in issue #9 as a test case.