Fermi docs example broken

The example in the [Fermi tutorial](http://astroquery.readthedocs.org/en/latest/fermi.html#getting-started) doesn't work:
```
In [22]: from astroquery import fermi

In [23]: query = fermi.FermiLAT_Query()
ERROR: TypeError: __call__() takes at least 2 arguments (1 given) [IPython.core.interactiveshell]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-23-6470dbf3fa96> in <module>()
----> 1 query = fermi.FermiLAT_Query()

TypeError: __call__() takes at least 2 arguments (1 given)
```

@jdnc Can you re-factor Fermi soon (see #112)?
If not I might have time to do it next week.