FIX: Ignore bad channels when dropping epochs

When bad channels are marked in a raw file, epochs shouldn't be rejected because of those channels. Another option is to restrict the data in epochs to only contain data from good channels (by modifying the default picks), but then users who use `pick_types` to pick channel types will have to remember to add `exclude=raw.info['bads']`, which is not nice. Also, I think the way inverses are structured, the bad channels are never used anyway, so this should be good.