adding anonymous access for StackableArray (see #52)

Seams I got this working somehow.  I did a lot of playing around with it, so please have a short look at it before merging to be sure I didn't break anything else. For example I didn't figure out what the 4th argument of pthreads_store_isset is surposed to be set to. 

Of course if you have any idea how to get this "nicer" done, than to add the counter into the pthreads storage, I'm glad to hear about it. Maybe some structure the counter can be added to and used with an atomic increment instead of using a lock there?