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

Feature request for the query builder:

``` go
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
