Allow passing in a handler

A handler is typically used to use alternate
providers of async functionality in kazoo, for
eventlet a specialized handler can be provided,
or the same for gevent; so it's nice to allow
the user to pass that in (the default if not
passed in will be a SequentialThreadingHandler
which as we know in python means threads, and
threads aren't such great performers due to
the GIL).

Change-Id: Iad928c26c6eeda1e735ae8e1b1f8124d0e45404f