check if browser supports AudioContext.close in webaudio.js (#962)
check if browser supports AudioContext.close in webaudio.js (#962)

add check for browser support of AudioContext.close(). This is only supported by Chrome 43+ and FireFox 40+ for now. Edge and other non supported browsers will give an undefined error.

https://developer.mozilla.org/en-US/docs/Web/API/AudioContext/close

// check if browser supports AudioContext.close()
            if(typeof this.ac.close() !== 'undefined'){
                this.ac.close();
            }

fixes #949 
agamemnus/wavesurfer.js,agamemnus/wavesurfer.js,ale-grosselle/wavesurfer.js,ale-grosselle/wavesurfer.js