Added function getDuration by request of Erik Oros.

The success call will now return the duration of a sound file.

For example:

PGLowLatencyAudio.getDuration("alien.wav", function(duration) {
    if (duration > 6) {
        // Body to do if duration is greater than 6
        alert("GREATER THAN 6"); // should alert true.
    } else {
        // Body to do if duration is less than 6
        alert("LESS THAN 6");
    }
});

Also changed up README.