(#227) allow for usage of non-blocking locks inside with statements

 Introduce an optional argument to __init__ allowing for specification of blocking behavior at instantiation while maintaining backwards compatibility.

This allows for use of non-blocking Locks inside with statements.

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```
zk = KazooClient()
with zk.Lock("/lockpath", "my-identifier", blocking=False) as lock:
    if lock.is_acquired:
        # do something
```
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