API: Should this raise an error or a warning in HDFStore?
Currently in 0.12.

```
In [3]: DataFrame(randn(10,2)).to_hdf('test_storer.h5','df')

In [4]: DataFrame(randn(10,2)).to_hdf('test_table.h5','df',table=True)
```

Selecting from a table is good with or without a where

```
In [6]: pd.read_hdf('test_table.h5','df',where='index>5')
Out[6]: 
          0         1
6  0.296947 -1.876055
7 -0.200516 -0.641670
8 -0.177828  0.877286
9  0.836903 -1.626247
```

This raises if where is not None, the theory being that passing a non-None where was a mistake.

However, then you have to a-prior know if you are dealing with a `Storer` or a `Table`

```
In [7]: pd.read_hdf('test_storer.h5','df',where='index>5')
TypeError: cannot pass a where specification when reading a Storer
```

make this a warning instead?
