Reset __len__ after bf.clear_all()

I expected that after bf.clear_all() calling len(bf) would give 0.  It looks like it isn't being re-set.
```
>>> bf = pybloomfilter.BloomFilter(100, .01)
>>> silent = bf.add('foo')
>>>  len(bf)
1
>>> bf.clear_all()
>>> len(bf)
1
```

I'm getting this for both in-memory and file-based filters, on Ubuntu and OSX.  Python 2.7 in each case.
