Changed the function to work with floats.

This is needed for future super-sampling implementation as this is the place
where the variability will be introduced.
Added += operator to Vector
Added /= operator to Vector
Removed RGB class to represent color

This is done in favor of the Vector class which will be used to represent
color. It is much easier to have just one class to represent all vector
objects, since we do not need to manage the same code twice or more.