Fixed RGB printing bug

Fixed a bug where the RGB class, while having the proper values
stored internally, printed them improperly. This is because the
RGB values were stored as `unsigned char`s. While this is correct,
it means that `std::cout` tries to interpret the values as characters.
By casting to `int`, the issue was resolved.

* Modified main.cpp - added test code for the RGB class.

* Modified RGB::print() - cast output values to int.