Added Color operators

Hi,
I have added the following operations to the `Color` struct:
* +, -, *, /
* +=, -=, *=, /=
* ==, !=

And also added `toString()` method.

I have tested them with the following unittest
```D
import dsfml.system;
import dsfml.window;
import dsfml.graphics;
import std.stdio;

void main()
{
	writeln("Testing");
}

unittest
{
	Color yellow = Color.Yellow;
	Color blue = Color.Blue;

	Color result = yellow + blue;
	yellow += blue;
	assert(result == Color.White);
	assert(result == yellow);
	assert(yellow != Color.Black);

	yellow /= Color(255, 8, 16, 32);
	writeln(yellow.toString());

	assert((Color.Yellow + Color.Blue) == Color.White);
}
```