Repeating Field Groups

Repeating field groups are currently wrapped with a div. This is fine, but not if you use table
rows for repeating, so I added a second parameter to the method which lets you overwrite the default div tag. In the loop it would be used as follows

$mb->the_group_open("tr", "tbody"); 

First argument is the element that is looped and the second is the wrapper of this repeating element