DataWatch is using TypeError to detect the arity of callback function

[liudangyi]

In kazoo/recipe/watchers.py line 163.

            try:
                result = self._func(data, stat, event)
            except TypeError:
                result = self._func(data, stat)

It's too terrible since a lot of error could raise a TypeError. Two better ways may be

1. Like ChildrenWatch, add a send_event parameter.
2. Use len(inspect.getargspec(self._func).args) to decide the arity first.