Clarify what the "advanced code" is and only output the advanced code snippet if the field has been filled out

Thanks for updating the module to work with Snapengage's new javascript. After looking over the module I had a couple of edits that I think clarifies the purpose of the new javascript and cleans up the output for those who do not need the advanced functionality.

Clarifications

1. Change the checkbox label and description for the "snapengage_widget_advanced" form element to reflect that there is a new version of snapengage's javascript. The rational for this is that there really is not two versions of the javascript. The googleapis script has replaced the old appspot snapabug.js script.
2. Change the description of the "snapengage_widget_advanced_script" text field to indicate that it is optional and if you don't need it you can just leave it blank.

Code Change

Add in an extra check to see if something is in the advance script field. If there isn't anything in the field then just load the script and don't print out the advanced javascript snippet as it isn't being used.

I will attempt to submit a patch with the proposed changes.