Fix for Issue #550

IE throws an error when calling insertBefore when beforeSlide is
undefined, other browsers do not.  I've added in an if/else to handle
the situation when beforeSlide is undefined so that the slide is still
added to the wrapper and prevents a JS error in IE.
Merge remote-tracking branch 'upstream/master'
Merge pull request #551 from Evidica/master

Fix for Issue #550