PyTables Light

Implements smoother datetime handling for into and provides a lightweight
wrapper around a `tables.Table` object.

closes #530

- [ ] handle `Time32Col`
- [ ] test that `into` appends to a pytables table when called twice on the same node
- [ ] test `into` with two tables as arguments
Historically I've used `into` only for data, and not for type conversions.  I tend to make functions like `pytables_to_numpy_dtype` instead.  Not sure if this is preferable or not.