Numpy testing

Tests can now be executed from within a(n) (I)Python session by doing
something like:

```python
>>> import skbio
>>> skbio.test()
.......... .....
XXX Tests OK
>>> skbio.parse.sequences.test()
........ .......
```
Notes:

The only odd thing is that if we add the few configuration lines as
suggested by Numpy's documentation, then we end up with an error like
this:

```python
>>> import skbio
>>> skbio.test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/yoshikivazquezbaeza/git_sw/scikit-bio/skbio/__init__.py", line 54, in test
    return Tester().test(level, verbosity)
  File "/Users/yoshikivazquezbaeza/.virtualenvs/skbio/lib/python2.7/site-packages/numpy/testing/nosetester.py", line 146, in __init__
    raise AssertionError
AssertionError
>>>
```

That's the reason why we are not doing it as the documentation suggests.

https://github.com/numpy/numpy/blob/master/doc/TESTS.rst.txt