Added support for loading .ogg audio files using a combination of
`SDL_RWFromConstMem()` + `Mix_LoadMUS/WAV_RW()`.

`SDL_RWFromConstMem()` now returns an opaque reference that corresponds
to a Javascript-side object which retains the information passed in.
The same for `SDL_RWFromFile()` but it saves different information.

When `Mix_LoadMUS/WAV_RW()` is called, it now expects a reference to
one of those Javascript-side objects and from it, either uses a
pre-loaded Audio, or reads bytes from the file-system to make one,
or uses the bytes originally passed into `SDL_RWFromConstMem()` to
create one.

Note that with this change you can now simply embed sound files in your
app, you do not need to preload them.
Updated with kripken's suggestions.
Made sure that `IMG_Load_RW` is also aware that it should receive
`SDL_RWops *` generated from `SDL_RWFromFile()`.
Formatting cleanup.
Added mime type information to SDL_rwops in `SDL_RWFromFile` as a hint
to processing down the line.

Changed the `Pointer_stringify()` call to occur in `SDL_RWFromFile`
instead of downstream which makes more sense.

Cleaned up formatting.
Made getMimetype() a public method of the Browser module and used it
inside the SDL module.
Merge pull request #1386 from michaeljbishop/sdl_rwops

Add support for `SDL_RWFromConstMem()` for .ogg files.