Join thread after close()

This prevents occasional errors on Python 2.7 when exiting the
program:

    Exception in thread Thread-1 (most likely raised during interpreter shutdown):
    Traceback (most recent call last):
        File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
        File "/usr/local/lib/python2.7/dist-packages/sqlitedict.py", line 255, in run
        File "/usr/lib/python2.7/Queue.py", line 179, in get
        File "/usr/lib/python2.7/threading.py", line 279, in notify
        <type 'exceptions.TypeError'>: 'NoneType' object is not callable

As far as I know, these errors are benign, but it is good practice to always join your threads.