Fix clamping on levels by adding Camera:setLevel

Setting the level is also available in the Camera constructor. If the
level is set, then autoclamping will be enabled. I've also added [opt]
to optional parameters. Instead of having set and remove for camera
instance variables, simply using a set function with nil value
(or nothing, as it will pass nil) will clear the instance variable.

Updated documentation, modules should use "Returns..." while classes
should use "Creates..." in their description (OCD level: masterful).