[VV] Agility

I want agile ships to be able to dodge, but high maneuverability in FT
doesn't help a ship evade shots.  So let's use Noam's stealth
mechanism (http://nift.firedrake.org/defences/WDA-ECM.htm ) and call
it "agility".

Not decided yet whether to follow Noam's proposal of 0mass / 2pts per
hull + armor, or to treat it as an extra level of Main Drive.