Added suppport for multiple compressors in Blosc 1.3.0

@esc Can you have a look at this?  Just to wet your appetite:

```
PYTHONPATH=. python bench/compare-pack-ptr.py 
Creating a large NumPy array with 10000000 int64 elements...
[      0       1       2 ..., 9999997 9999998 9999999]
Using *** blosclz *** compressor
Time for pack_array/unpack_array:     0.140 s.  Compr ratio: 136.24
Time for compress_ptr/decompress_ptr: 0.051 s.  Compr ratio: 136.83
Using *** lz4 *** compressor
Time for pack_array/unpack_array:     0.136 s.  Compr ratio: 136.73
Time for compress_ptr/decompress_ptr: 0.051 s.  Compr ratio: 137.19
Using *** lz4hc *** compressor
Time for pack_array/unpack_array:     0.257 s.  Compr ratio: 164.97
Time for compress_ptr/decompress_ptr: 0.155 s.  Compr ratio: 165.12
Using *** snappy *** compressor
Time for pack_array/unpack_array:     0.145 s.  Compr ratio: 20.36
Time for compress_ptr/decompress_ptr: 0.060 s.  Compr ratio: 20.38
Using *** zlib *** compressor
Time for pack_array/unpack_array:     0.410 s.  Compr ratio: 406.45
Time for compress_ptr/decompress_ptr: 0.309 s.  Compr ratio: 407.60
```

:)