update the Bloscapck plug

I have done some profilings on the decompression part, and I don't see any special overhead in cStringIO.  Rather, I think the problem is more the adler32 checksum.  After disabling the use of checksums, you get decompression speeds that are comparable to compression:

```
In [50]: a = np.linspace(0, 1, 1e8)

In [51]: %time c = blp.pack_ndarray_str(a, blosc_args=blp.BloscArgs(clevel=9, cname='lz4'), bloscpack_args=blp.BloscpackArgs(checksum="adler32"))
CPU times: user 1.23 s, sys: 59.7 ms, total: 1.29 s
Wall time: 577 ms

In [52]: %timeit b = blp.unpack_ndarray_str(c)
1 loops, best of 3: 617 ms per loop

In [53]: %time c = blp.pack_ndarray_str(a, blosc_args=blp.BloscArgs(clevel=9, cname='lz4'), bloscpack_args=blp.BloscpackArgs(checksum="None"))
CPU times: user 1.08 s, sys: 50.6 ms, total: 1.13 s
Wall time: 474 ms

In [54]: %timeit b = blp.unpack_ndarray_str(c)
1 loops, best of 3: 446 ms per loop

In [55]: %time ap = cPickle.dumps(a, protocol=-1)
CPU times: user 411 ms, sys: 250 ms, total: 660 ms
Wall time: 664 ms

In [56]: %timeit cp = cPickle.loads(ap)
1 loops, best of 3: 570 ms per loop
```

Notice how compression goes faster too.  And both are faster than cPickle in this case.

Given this, I would disable the checksum `pack_ndarray_str()`, because it has little sense to use it for in-memory purposes (unless you want the outcome to transfer it through the net, but in this case you have the typical checksum implemented at IP level, and in any case, the user can always manually activate it).

Also, I suggest to use ``None`` (the Python object) instead of 'None' (the string) for deactivating the checksum.