Adding an auto FOV system

Closes #4

Adds [g|s]etDefaultFOVCallback which provides a way to override the default
FOV scaler system in place.
getFOV now uses defaultFOVCallback to calculate a FOV if none is set
getFOV now requires width and height arguments unless FOV is already set.
defaultFOVCallback uses a comfortable 90 degrees of vision instead of the
very common 70 degrees, but if you look at the function defined in level.new,
one will notice it is incredibly easy to override.
The no longer has FOV scaling inputs, and uses this new system.
Removing docs for FOV scaling inputs