WebAudio bindings in Fable.Import.Browser are incomplete or don't use ideal types.

[JohnStov]

Some methods are missing. Many take floats where ints would be more meaningful. I'm happy to tidy this up and add some StringEnums - just want to check that it's the right thing to do.

[alfonsogarciacaro]

@JohnStov Sure! It'll be great if you can help improve the bindings for WebAudio. I just merged PR #40 to improve the browser bindings by using latest ts2fable with latest TypeScript DOM lib, but there was nothing about WebAudio (it was mainly to remove Func and add comments) so it probably needs some manual tweaking.