Support JOINs with _SetOperation subqueries (union, itersect, except, minus)

[foggel]

Joining with a union subquery fails with: ValueError: Cannot join on type '<class 'pypika.queries._SetOperation'>'

Example:

        subquery = (
            Query.from_(self.table_abc).select("*")
            .union(
                Query.from_(self.table_abc).select("*")
            ).as_("subq")
        )

        test_query = (
            Query.from_(self.table_abc)
            .join(subquery)
            .on(subquery.x == self.table_abc.id)
            .select(self.table_abc.foo)
        )

[coveralls]

Coverage increased (+0.002%) to 98.174% when pulling 1f6ab34 on foggel:master into 75255d2 on kayak:master.