Spark SQL basicOperators  add Except operator

Hi all,
I want to submit a Except operator in basicOperators.scala
In SQL case.SQL support two table do except operator.
select * from table1
except
select * from table2 
This operator support the substract function .Return an table with the elements from `this` that are not in `other`.This operator should limit the input SparkPlan Seq only has two member.The check will later support
Maybe name this operators `Subtract`.  In general most of the catalyst/spark SQL operators are named after the relational operations they are performing.

If you aren't using `sc` I'd drop it and the `otherCopyArgs`.

Also, lets enforce the constraint from the TODO below using the typesystem.  Just have the operator be a BinaryNode with two children, `left` and `right`.