fix list sorting and add test

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ERROR: test_sort (test.tests.TestListProperty)
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Traceback (most recent call last):
  File "/Users/droberts/jsonobject/test/tests.py", line 698, in test_sort
    foo.list.sort()
  File "/Users/droberts/jsonobject/jsonobject/containers.py", line 69, in sort
    zipped.sort(reverse=reverse)
AttributeError: 'itertools.izip' object has no attribute 'sort'