Support += (__iadd__) on list properties

[dannyroberts]

currently

In [1]: from jsonobject import *

In [2]: class Foo(JsonObject):
   ...:     bar = ListProperty()
   ...:     

In [3]: foo = Foo()

In [4]: foo.bar = [1, 2, 3]

In [5]: foo.to_json()
Out[5]: {'bar': [1, 2, 3]}

In [6]: foo.bar += [4]

In [7]: foo.to_json()  # The change isn't reflected here
Out[7]: {'bar': [1, 2, 3]}

In [8]: foo.bar  # ...even though it is here
Out [8]: [1, 2, 3, 4]