Made a "rotate conveyor" command

Also made a visual indicator of the direction a
   conveyor is facing
fixed possible bug in the conveyors
improved get_building_tile

made it use a laser and be more in line with the
   digging; this was a major user-experience
   improvement for me
neatened up some unit conversions
Made uphill conveyors friendlier
Merge branch 'master' of https://github.com/idupree/Lasercake

Conflicts:
	specific_object_types.hpp
sped up the mouse rotation
switched WASD keys to four-directional movement
Added some mouse acceleration; feels better to me
Fixed the frustum edge glitch
updated hud text, made it a bit more consise
made rubble less transparent and patterned like rock
Fixed obvious bug in autorobots
add comment to frustum checking code
eliminate units_tests.cpp warnings (for GCC 4.7 at least)
improve units error messages / orthogonality a bit.
Made metal.

Each tile has an initial amount of metal in it
That quantity is carried wherever we move the rubble
The only thing that changes it is the refinery
Tiles with more metal show up greener
remove now-unnecessary high rounding error fixing value

now that we fixed the reason it was hackily high
Merge branch 'master' of https://github.com/elidupree/Lasercake