Problem: impossible to get at keys of key/value pairs in a zing

Solution: provide zring_key() method:

//  Return the key of the current item in the ring. If the ring is empty, or the
//  cursor passed the end of the ring, returns NULL. Does not change the cursor.
CZMQ_EXPORT const void * zring_key (zring_t *self);
Merge pull request #791 from skaes/add_zring_key_method

Problem: impossible to get at keys of key/value pairs in a zring