Can't ignore warnings

I can't ignore warnings using the instructions here: http://docs.astropy.org/en/v0.1/io/fits/usage/misc.html#warning-messages.  However, I think that page should be made *much* more visible, maybe even with a link on the homepage, but I really wanted it to show up on a google search for "ignore warnings astropy".

```
In [1]: import astropy.units as u
WARNING: ConfigurationDefaultMissingWarning: Requested default configuration file /Users/adam/repos/astropy/astropy/astropy.cfg is not a file. Cannot install default profile. If you are importing from source, this is expected. [astropy]

In [2]: x = np.linspace(0,3)*u.pc

In [3]: plot(x)
WARNING: Converting Quantity object in units 'pc' to a Numpy array [astropy.units.quantity]
Out[3]: [<matplotlib.lines.Line2D at 0x100635850>]

In [4]: import warnings

In [5]: warnings.filterwarnings('ignore', category=u.UnitsWarning, append=True)

In [6]: plot(x)
WARNING: Converting Quantity object in units 'pc' to a Numpy array [astropy.units.quantity]
Out[6]: [<matplotlib.lines.Line2D at 0x100642410>]

In [7]: import astropy

In [8]: astropy.__version__   # using python setup.py develop
Out[8]: '0.3.dev3273'

In [9]: warnings.filterwarnings('ignore') # I thought this would ignore ALL warnings?

In [10]: plot(x)
WARNING: Converting Quantity object in units 'pc' to a Numpy array [astropy.units.quantity]
Out[10]: [<matplotlib.lines.Line2D at 0x10063dad0>]

```