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
I've also been putting things like this straight into `/blaze` rather than `/data`.  It might make sense to move them, all to one place or the other.