fix setbit

[cosven]

1. setbit SHOULD return old_value instead of None
2. there exists bug when setbit to 0

for example:

# originally
self.redis.setbit('foo', 4, 0)
value = self.redis.getbit('foo', 4)  # we will get 1, but we should get 0

[adamantike]

I have experienced this same issue. The simplest scenario where current implementation fails is:

def test_getbit_zero(self):
    self.redis.setbit('foo', 1, 0)
    self.assertEqual(self.redis.getbit('foo', 1), 0)

@jamesls can you please consider merging this fix?

[mrnovalles]

Could you release a patch version now that this fix is merged?