Sound playback improvements - added optional offset and duration parameters

[mikedotalmond]

Adds ability to play or loop regions of a sound, with optional offset and duration parameters.

play (volume :Float = 1.0, offset:Float=0, duration:Float=0) :Playback
loop (volume :Float = 1.0, offset:Float=0, duration:Float=0) :Playback

This will allow sound-sprite type behaviours - where many short sounds can be contained in, and played from, a single asset.

The update should be seamless across all platforms - and not break anything.. but I have changed how Playback objects are disposed:

• There are changes to the onComplete behaviour of one-shot Playback objects: once complete, the Playback is disposed + released for GC. The previous behaviour required users to dispose of Playback manually once a sound completed in order for it to be culled.
  In the case of the WebAudioPlayback, nodes were remaining in the audio graph after sounds completed. So playing lots of one-shot sounds created many AudioBufferSourceNode -> GainNode pairs that were not being removed/collected.
• Some Dynamic properties in the html audio platform classes have also been changed to use the typed js.html and js.html.audio defs
  Note: I had to update the signatures for AudioNode::connect and AudioNode::disconnect which are out of date in the haxe std library. The output and input parameters for these functions are optional so should be more like this...
  @:overload( function( destination : AudioNode, ?output : Int, ?input : Int ) :Void {} )
function connect( destination : AudioParam, ?output : Int ) : Void;
function disconnect( ?output : Int ) : Void;

[mikedotalmond]

Ah, I see I didn't update Mixer. Will fix.

[mikedotalmond]

Hmm, I see why those js.html.audio externs were left out... revert....

[mikedotalmond]

Build appears to have failed on a firefox.js test - timed out after 10 mins. Not sure why.
https://travis-ci.org/aduros/flambe/jobs/32797267