Proxied invocation of overridable functions through call to apply `this` context

Allows overridden `custom` upload/error functions to access the inlineattach object for calling back on the `onErrorUploading`/`onUploadedFile` functions.

i.e.
```
$('.inlineattach').inlineattach({
  customUploadHandler: function(file) {
        var that = this;
        MyCustomUploader(function(error, data) {
            if (error) {
                that.onErrorUploading();
            } else {
                that.onUploadedFile(data);
            }   
        });
  }});
```