Fixed errors in decoding SDL_MouseButtonEvent.button

* `peek` is guided by types; gave the correct type for the field.
* The field is apparently not limited to the documented values (reported to SDL as bug 2310).
Merge pull request #18 from rhymoid/master

Fixed errors in decoding SDL_MouseButtonEvent.button
Fixed decoding of scancode and keycode in SDL_Keysym.
Merge pull request #19 from rhymoid/master

Fixed decoding of scancode and keycode in SDL_Keysym.
add alpha vlaues to Color