BUG: empty result for HDFStore where with leading NaNs
https://github.com/PyTables/PyTables/issues/282 is the culprit 

I may be missing something here, but I think this is an odd result. When reading from a `table` HDFStore, if the first value is NaN then any `where`s based on that column will return an empty selection.

``` python
In [58]: sm
Out[58]: 
            HRHHID  earnings
0  251395039600155       NaN
1  251396749500840    188461
2  251396749500840     57692
3  251408550479107       NaN
4  251465059600832     61500

In [63]: tst = pd.HDFStore('tst.h5')

# leading NaN, empty
In [64]: sm.to_hdf(tst, 'sm', data_columns=True, format='table')

In [65]: tst.select('sm', where="earnings>0")
Out[65]: 
Empty DataFrame
Columns: [HRHHID, earnings]
Index: []

# leading isn't NaN, works
In [66]: sm.tail(4).to_hdf(tst, 'sm', data_columns=True, format='table')

In [67]: tst.select('sm', where="earnings>0")
Out[67]: 
            HRHHID  earnings
1  251396749500840    188461
2  251396749500840     57692
4  251465059600832     61500

# Leading NaN again, empty
In [69]: sm.tail(2).to_hdf(tst, 'sm', data_columns=True, format='table')

In [70]: tst.select('sm', where="earnings>0")
Out[70]: 
Empty DataFrame
Columns: [HRHHID, earnings]
Index: []


In [71]: pd.__version__
Out[71]: '0.12.0-1153-g476b6e3'

In [72]: import tables

In [73]: tables.__version__
Out[73]: '3.0.0'
```
