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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This file has a half-dozen optional dependencies.  We're often testing interactions between them, e.g., can I turn a PyTables dataset into a MongoDB collection?  This process should be robust to a few missing dependencies; we'll still want to test the others.  `skip_if_not` is the cleanest way I know how to accomplish this for now.