rewrote autoplacement code to make it more aware of filters,
autocrafter, nodebreaker, deployer, and made sure each item will only
initiate a connection to those sides which can accept such.  Fixed
various autorouting bugs as I ran across them.  Autorouting for various
devices is now:

filters: left and right sides only
nodebreaker, deployer:  back only
autocrafter:  all six sides
chests:  top, bottom, left, right, back (not front)
furnace: bottom, left, right, back (not the top or front)
actually, filters should only connect from the right/output side since they
can't actually receive anything from the input side via a tube.
Update Overview.html with documentation about the node breaker and deployer, plus docuent the fact that tubes only connect to certain sides of certain devices, like injectors and furnaces.
Node breakers now make sure the node is actually diggable before digging (implements the functionality of `minetest.node_dig`). Now can dig working mesecons and can't dig locked chests or other indestructable nodes.