Fix `ImportError: No module named numpy` when numpy module not installed

When importing `blosc` in environment without `numpy` i receive exception:

```
>>> import blosc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/mnt/data/Work/trash/tests/lib/python2.7/site-packages/blosc/__init__.py", line 46, in <module>
    from blosc.test import run as test
  File "/mnt/data/Work/trash/tests/lib/python2.7/site-packages/blosc/test.py", line 5, in <module>
    import numpy
ImportError: No module named numpy
```

I make `numpy` module optional in `blosc.test`.