Add ability to iterate all documents in a tailable cursor in a single for loop.

Iterating all documents in a tailable cursor currently requires multiple for loops:

while tailable_cursor.alive:

    for doc in tailable_cursor:

        print(doc)

    print('getMore returned 0 documents') 

It would be nice if there was a way to iterate all the documents in a single loop:

for doc in tailable_cursor:

    print(doc)

However, we can't make this the default behavior because applications may rely on breaking out of the loop.