Merge pull request #133 from Novatux/master

Use swap_node instead of add_node when replacing conductors.
Remove command block until #136 is fixed
First draft of some kind of Action Queue (just like the globalstep queue in to_update), but more flexible and also including delay functionality (mesecon_delayer).
The queue is also saved to a file, so that when restarting mesecons, delayers resume to the state they had when the game shut down. Needs testing.
Fix the bugs spotted by Novatux - thanks for spotting them
Add ActionQueue priority system
This makes effectors nearer to the source of the action (the receptor) update first.

This defines behaviour for this piston circuit: http://i.imgur.com/9Pp2Mzb.png
And defines, that this memory circuit does not work from this direction: http://i.imgur.com/jJn0aFh.png
But it will work when using the switch from the other side: http://i.imgur.com/nvw0oZB.png

Only if two effectors have the same distance, there is nothing we can do about it, behaviour is not defined.
"Distance" is determined by the stack size of recursions in turnon / turnoff.
Priorities are between 0 (lowest) and 1 (highest).
Fix ActionQueue delays
Fix unloaded area in receptor_off, yet it was only fixed in receptor_on
Resume turnon/off calls as soon as area is loaded in case turnon/off calls end in unloaded territory
Fix infinite priority bug in mesecon:turnoff, thanks to Novatux
Turnon/turnoff overwritable