fix getDuration() return value when using MediaElement (#941)

* fix getDuration() return value for MediaElement 

change getDuration function in mediaelement.js to return the correct duration length same as WebAudio.

from
        var duration = this.media.duration; // incorrect duration value returned
        console.log preview:
        https://cloud.githubusercontent.com/assets/5193884/22399178/6d26c1fe-e565-11e6-9b13-e6107641666a.png
to
        var duration = this.buffer.duration; // correct duration value returned same as WebAudio
        console.log preview:
        https://cloud.githubusercontent.com/assets/5193884/22393549/7b130096-e4d6-11e6-83ff-4ebb78b9e42f.png

* Update mediaelement.js

src/mediaelement.js

@@ -125,7 +125,7 @@ WaveSurfer.util.extend(WaveSurfer.MediaElement, {
     },
 
     getDuration: function () {
-        var duration = this.media.duration;
+        var duration = (this.buffer || this.media).duration;
         if (duration >= Infinity) { // streaming audio
             duration = this.media.seekable.end(0);
         }