Booleans

Add `And` and `Or` binary operators to table Expressions.  Add tests to ensure that they work in the three backends.

Note that I'm only doing binary operator booleans.  This is because I want to avoid multiple inheritance in the TableExpr layer (multiple inheritance and multiple dispatch are hard to do simultaneously.)  I believe that this will be simpler to do in the Scalar expr layer if we go down that route.

CC @talumbau

<!---
@huboard:{"order":291.5,"custom_state":""}
-->