update the Bloscapck plug

Also, with larger arrays:

```
In [49]: a = np.arange(5e7)

In [50]: %timeit ap = cPickle.dumps(a, protocol=-1)
1 loops, best of 3: 536 ms per loop

In [51]: ap = cPickle.dumps(a, protocol=-1)

In [52]: %timeit cp = cPickle.loads(ap)
1 loops, best of 3: 348 ms per loop

In [53]: %timeit 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))
10 loops, best of 3: 63.6 ms per loop

In [56]: 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))

In [57]: %timeit bpu = bp.unpack_ndarray_str(bpc)
10 loops, best of 3: 120 ms per loop
```