Add Keyboard class for handling raylib keyboard related functions

[swiderskis]

I noticed there are classes defined for handling mouse (Mouse.hpp) and gamepad (Gamepad.hpp) inputs. However, there are also keyboard input functions in raylib that are not defined in raylib-cpp.

I think it would make sense to define these in a Keyboard class. If you agree, would you like me to make a PR for this?

[swiderskis]

Also, perhaps the SetExitKey() function should be added as a method of the Window class?

[RobLoach]

yaaaaaaaaaaaaaaaaaaaaaaaaaaaaaas

[swiderskis]

Added in #275