Added a callback function to allow for dynamic tooltip generation.

It should be possible to do:
content : function(xval, yval){
   if(xval==1234){
       return '<div>'+yval*2+'</div>';
   } else {
       return '<div>'+yval*yval+'</div>';
   }
}
Merge pull request #12 from fmsf/master

Callback function to allow for dynamic tooltip generation