jshint fixes

Here are some jshint fixes. There are many more but I'm not sure if we want to fix them (style-related). I suggest you also run jshint on the javascripts and see if there are any others we want to fix.
Hmm, these changes in the `seekTo` method cause the following error:

```
TypeError: Argument 3 of AudioBufferSourceNode.start is not a finite floating-point value.
http://localhost:8080/wavesurfer.js/src/webaudio.js
Line 297
```
So I guess just blindly following jshint's advice is not a good idea or there's a bug in `seekTo` that I'm not seeing.