Utility function for making tables a little easier

I just hammered this out really quickly, but it's a little sad because it's using concatenation to build xpath locators, and so is restricted to xpath only.

I think a better solution would be to change Table's initializer to have the `(loc, header_offset, row_offset)` signature, but that would require all tables used by this class to have `thead` and `tbody` sections. I think that they all *should*, and haven't seen an example to the contrary yet, and this would make Table less of a pain to use.