Add keycodes to Enum
Add Enum's for `SDL_AUDIO_ALLOW_XXX_CHANGE`
Add .gitignore file
Update cabal file to version 1.1.3
add the missing convenience keymods
Add InitFlag and make the flags non-polymorphic
Make key modifier constants polymorphic

The SDL2 library doesn't use them consistently; for example in the
SDL_Keysym struct, they're referred to as a Uint16.
Remove Keycode export from Types

We no longer need the backwards-compatibility.