Add Remaining IsKey___() Functions

[RobLoach]

This adds the three remaining IsKey____() functions...

IsKeyPressedRepeat(key)
IsKeyReleased(key)
IsKeyUp(key)

There aren't any official raylib examples that use these functions, so I avoided adding them as examples. It can be tested, however, by adding the following to core_input_keys.c, and pressing R to reset the ball's position.

if (IsKeyReleased(KEY_R)) {
    ballPosition.x = (float)GetScreenWidth()/2;
    ballPosition.y = (float)GetScreenHeight()/2;
}