HDFStore.append_to_multiple doesn't write rows that are all np.nan
Hi,

Using HDFStore.append_to_multiple, if an entire row written to any one table consists entirely of np.nan, the row is not written to the table, but is written to the other tables.  The following code reproduces and fixes the issue.  

I would prefer that append_to_multiple maintain synchronized rows across tables, and to my knowledge, the best way to do that is drop that row from the other tables.  We would probably need a fix that looks something like this PR.

I'm not sure if this is the best way to do this, and would love some feedback!

Thanks, 
Alex

The Fix:   

```
# setup
d = {'table1': ['col1', 'col2'], 'table2': ['col3', 'col4']}
value = pandas.DataFrame([[1,2,3,4], [5,6,None, None]], columns=['col1', 'col2', 'col3', 'col4'])

# fix (as in PR)
dfs = (value[cols] for cols in d.values())
valid_index = reduce(
        lambda index1, index2: index2.intersection(index1),
        [df.dropna(how='all').index for df in dfs]) 
value = value.ix[valid_index]
```

To reproduce the error:

```
In [1]: from pandas import DataFrame, HDFStore

In [2]: store = HDFStore('tmp1.h5')

In [3]: df = DataFrame([[1,2], [3,None]], columns=['a', 'b'])

In [4]: store.append_to_multiple({'table1': ['a'], 'table2': ['b']}, df, 'table1')

In [5]: store
Out[5]:
<class 'pandas.io.pytables.HDFStore'>
File path: tmp1.h5
/table1            frame_table  (typ->appendable,nrows->2,ncols->1,indexers->[index],dc->[a])
/table2            frame_table  (typ->appendable,nrows->1,ncols->1,indexers->[index])
store.select_as_multiple(['table1', 'table2'])

In [6]: store.select_as_multiple(['table1', 'table2'])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-1a2861fcaa34> in <module>()
----> 1 store.select_as_multiple(['table1', 'table2'])

/Users/alexgaudio/.virtualenvs/ds/lib/python2.7/site-packages/pandas/io/pytables.pyc in select_as_multiple(self, keys, where, selector, columns, start, stop, iterator, chunksize, auto_close, **kwargs)
    540                 nrows = t.nrows
    541             elif t.nrows != nrows:
--> 542                 raise ValueError("all tables must have exactly the same nrows!")
    543
    544         # select coordinates from the selector table

ValueError: all tables must have exactly the same nrows!
```
