Store "time to next pulse" instead of "timer start time"

That way the gate happily survives world time changes when it's chunk is unloaded without reseting.
renamed tag "ptonext" to "pelapsed"

Agreed, now - start is elapsed.
Merge pull request #94 from Iree/patch-1

Store "time elapsed" instead of "timer start time"
Basic tube framework

- Added tick sounds to all gates that need it.