add into(DataFrame, pytables Table)

An addition to `into` so that one can go directly from a pytables Table to a pandas DataFrame.

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Suggest breaking this up into the following two steps:

```Python
@dispatch(np.ndarray, table.Table)
def into(x, tb):
    return tb[:]

@dispatch(DataFrame, table.Table)
def into(df, tb, **kwargs):
    return into(df, into(np.ndarray, tb), **kwargs)
```