ENH: Adding '.' as an na_value for FRED.

The St. Louis Fed's [FRED](http://research.stlouisfed.org/fred2/) uses '.' as an NaN marker.  I don't think the user has any way to specify the na_value via DataReader, so this just hard codes '.' as an na_value in the fred data-fetching function.

Before:

```python
In [4]: from pandas.io.data import DataReader

In [5]: df = DataReader('DFII5', data_source="fred")

In [6]: df.head()
Out[6]:
           DFII5
DATE
2010-01-01     .
2010-01-04  0.52
2010-01-05  0.44
2010-01-06  0.44
2010-01-07  0.43
```

After
```python
In [2]: from pandas.io.data import DataReader

In [3]: df = DataReader('DFII5', data_source="fred")

In [4]: df.head()
Out[4]:
            DFII5
DATE
2010-01-01    NaN
2010-01-04   0.52
2010-01-05   0.44
2010-01-06   0.44
2010-01-07   0.43

In [5]: pd.__version__
Out[5]: '0.11.0rc1'
```

Hopefully I've done everything correctly with the pull request.  Let me know if anything needs fixing.