Updates the str_clean function for better safety.

This change will resolve #30 by insuring that any attempts to add customVars with quotes or slashes in the string have those characters properly escaped so they can be sent to Google Analytics without breaking the tracking code. This can be a serious problem if titles or category names are being sent that contain those characters (most commonly, if they contain an apostrophe). 

Without this change, the characters can appear un-escaped and potentially break tracking on the entire page. 

I wrote a little more about this issue at http://stackoverflow.com/questions/21686260/should-i-be-using-html-entity-decode-to-escape-a-google-analytics-custom-variabl