Feature: store.select should warn about unimplemented operators

Currently, when doing this (with object dtypes)

``` python
store.select('df', 'x != none') #this is not NaN, but the string 'none'
```

one receives exactly the same object as when doing

``` python
store.select('df', 'x = none')
```

The suggestion is that the query parser warns about the unknown or unimplemented operator.
