print help if no keyspace specified

Script currently crashes if invoked with no parameters:

$ python range_repair.py
Traceback (most recent call last):
  File "range_repair.py", line 372, in <module>
    main()
  File "range_repair.py", line 366, in main
    if repair(options):
  File "range_repair.py", line 299, in repair
    r.get()
  File "/usr/local/lib/python2.7/multiprocessing/pool.py", line 528, in get
    raise self._value
TypeError: sequence item 4: expected string, NoneType found