Query builder for .Union() and .UnionAll()

[VojtechVitek]

Feature request for the query builder:

res1 := col.Find(db.Cond{"name": "Max"}).Limit(10).Sort("-last_name")
res2 := col.Find(db.Cond{"name": "Rex", "age >": "21"}).Limit(15).Sort("-last_name")

err := db.Union(res1, res2).All(&people)

Related: #105

[pkieltyka]

I think Union() is a good thing to add to the SQL query builder, but I think in upper the generic builder via .Find() should be reserved to a subset of features just for doing the most basic conditions and using the sql query builder for all advanced queries, which is where Union would go.

[xiam]

Now that the query builder supports query expansion we can add Union()/UnionAll() methods to the builder and do something like:

sel1 := sess.SelectFrom()...
sel2 := sess.SelectFrom()...
sess.Union(sel1, sel2)

[xiam]

I think we can make this work with db.Result too, as you suggested @VojtechVitek. Let's do both.

[dencoded]

@xiam Would be interesting to see if this feature can be implemented in the near future. I think it is worth to add Union()/UnionAll() support to SQL-builder as it seems to be low level/custom SQL manipulation. The result would be the same Selector interface with all cool things like pagination.

Also, seems to be logical to add Intersect/IntersectAll and Minus/MinusAll (aka EXCEPT in postgres) to have all result set operation in SQL-builder.