DataFrame does not work well with quantities
``` python
from pandas import DataFrame
import quantities as pq
df = DataFrame([[pq.Quantity(i, pq.s)] for i in range(3)])
df[1:] - df[:-1]
```

raises: `ValueError: Unable to convert between units of "dimensionless" and "s"`

Note that the following works w/o error because pandas "strips" dimension:

``` python
df = DataFrame(pq.Quantity(np.array([0,1,2]).T, pq.s))
df[1:] - df[:-1]
```
