Physics, clouds, Level Gen'

Made a few tweaks to the physics without busting them, they're now
working even better with reliable values. Now all stage data such as air
density and atmosphere in general is stored in a Planet class; yup, I
decided to make Dragon Drop interplanetary. Why not? Air density grows
exponentially up to a maximum ground level value, which obviously comes
into effect on the movement possibilities of the player. There's a
slight hitch in the exp' equation that needs a fix, but that shouldn't
take too long.

Also added some scenery so the player can see movement. Clouds are
randomly generated and are destroyed when they leave the bounds of the
camera. Will change the spawns to occur more in certain atmospheric
conditions etc. at a later date.

Jobs for the next edit in a nutshell are to fix the exp equation so that
physics can be put to rest for a while, give the clouds some conveyance
by limiting them in areas outside the Thermosphere (i think that's the
one), and finally getting started on the obstacles. Also a couple minor
changes, such as an increase in lift when passing through a cloud, and
some extra controls like double taps for barrel rolls etc.