Add ColumnAs for 'SELECT col AS name' queries. (#24)

Add As helper for 'SELECT col AS name' queries

Clients could do this hackily before by manually passing "col AS name",
but it isn't clearly supported.

qb/select_test.go

@@ -28,6 +28,11 @@ func TestSelectBuilder(t *testing.T) {
 			B: Select("cycling.cyclist_name").Columns("id", "user_uuid", "firstname"),
 			S: "SELECT id,user_uuid,firstname FROM cycling.cyclist_name ",
 		},
+		// Add a SELECT AS column
+		{
+			B: Select("cycling.cyclist_name").Columns("id", "user_uuid", As("firstname", "name")),
+			S: "SELECT id,user_uuid,firstname AS name FROM cycling.cyclist_name ",
+		},
 		// Basic test for select distinct
 		{
 			B: Select("cycling.cyclist_name").Distinct("id"),