periodically remove one water node next to each "on" pump that's also flowing

This should make it work more or less correctly with finite water.

also, fix a bug where a blocked spigot still looks like its pouring, and check
for blockages on every step.
remove water node at spigot only at the
pouring-->off transition, not repeatedly.
made valves and airtight entry panels work.

Removed "flowing" spigot from creative inventory.
Make pumps and valves respond to mesecons signals
add flow sensor.  Sends mesecons signal when water is flowing through it.
Merge pull request #31 from Uberi/master

Update the Basic Overview page for the latest changes.
Fix the teleport tube in the latest mt.