update the Bloscapck plug

And now, also using ``%time``:

```

In [1]: import cPickle

In [2]: import bloscpack as bp

In [3]: a = np.arange(5e7)

In [4]: %time ap = cPickle.dumps(a, protocol=-1)
CPU times: user 260 ms, sys: 280 ms, total: 540 ms
Wall time: 540 ms

In [5]: %time cp = cPickle.loads(ap)
CPU times: user 192 ms, sys: 160 ms, total: 352 ms
Wall time: 350 ms

In [6]: %time bpc = bp.pack_ndarray_str(a, chunk_size='20M', blosc_args=bp.BloscArgs(cname='lz4'), bloscpack_args=bp.BloscpackArgs(checksum='None', offsets=False, max_app_chunks=0))
CPU times: user 208 ms, sys: 4 ms, total: 212 ms
Wall time: 68 ms

In [7]: %time bpu = bp.unpack_ndarray_str(bpc)
CPU times: user 128 ms, sys: 128 ms, total: 256 ms
Wall time: 104 ms
```

Seems to match the ``%timeit`` somewhat...