Add new RedshiftScheme constructor taking a conditions String

Rebased #8 on wip-2.5 branch

I couldn't find a way to restrict a Redshift tap to run over a subset of a table, so I added a constructor to RedshiftScheme to allow it to take conditions that will be added as a `where` clause to the query.

If there's an existing or better way to do this, I'd be happy to find it out. Otherwise this may be useful to others.