preventing duplication of audio context

This is related to discussion occurring in https://github.com/bbcrd/peaks.js/issues/69#issuecomment-55575226

When reusing the audio context in a single page application, new audio contexts are getting created over and over (actually 2 per load inside of peaks.js right now)

After a few loads an error like this appears:
`   Error: Failed to construct 'AudioContext': number of hardware contexts reached maximum (6).
    at Error (native)
    at Function.fromAudioObjectBuilder.getAudioContext (http://127.0.0.1:8081/dist/peaks.min.js:19:811)
    at Object.getRemoteData (http://127.0.0.1:8081/dist/peaks.min.js:46:804)
    at Object.init (http://127.0.0.1:8081/dist/peaks.min.js:46:403)
    at Function.Peaks.init (http://127.0.0.1:8081/dist/peaks.min.js:34:2471)
    etc...
`

Also, see this SO discussion of the error: http://stackoverflow.com/questions/25046470/failed-to-construct-audiocontext-number-of-hardware-contexts-reached-maximum