Add another stack fix

#237 

Functionality to add another item on top of the stack. 

@danielbachhuber @mboynes Please provide some feedback. 
I'd prefer to see the test confirming the position of the button relative to the meta boxes in addition to the added attribute, just in case something else down the line messed with it somehow. I'd also prefer to see the default (eg, current) behavior tested in addition to the test of the added behavior.

And since I'm nitpicking anyway, style-wise I prefer use of single quotes for increased readability in a context like this, eg: `'value="Add Another"'`