Support IN condition against subquery

Closes #94

Signed-off-by: Stephen Celis <stephen@stephencelis.com>

SQLite Tests/ExpressionTests.swift

@@ -536,6 +536,11 @@ class ExpressionTests: SQLiteTestCase {
         AssertSQLContains("(\"age\" IN (20))", contains(Set([20]), age))
     }
 
+    func test_containsFunction_withValueExpressionAndQuery_buildsInExpression() {
+        let query = users.select(max(age)).group(id)
+        AssertSQLContains("(\"id\" IN (SELECT max(\"age\") FROM \"users\" GROUP BY \"id\"))", contains(query, id))
+    }
+
     func test_plusEquals_withStringExpression_buildsSetter() {
         users.update(email += email)!
         users.update(email += email2)!

SQLite/Expression.swift

@@ -769,7 +769,6 @@ public typealias Star = (Expression<Binding>?, Expression<Binding>?) -> Expressi
 public func * (Expression<Binding>?, Expression<Binding>?) -> Expression<()> {
     return Expression(literal: "*")
 }
-
 public func contains<V: Value, C: CollectionType where C.Generator.Element == V, C.Index.Distance == Int>(values: C, column: Expression<V>) -> Expression<Bool> {
     let templates = join(", ", [String](count: count(values), repeatedValue: "?"))
     return infix("IN", column, Expression<V>(literal: "(\(templates))", map(values) { $0.datatypeValue }))
@@ -777,6 +776,9 @@ public func contains<V: Value, C: CollectionType where C.Generator.Element == V,
 public func contains<V: Value, C: CollectionType where C.Generator.Element == V, C.Index.Distance == Int>(values: C, column: Expression<V?>) -> Expression<Bool> {
     return contains(values, Expression<V>(column))
 }
+public func contains<V: Value>(values: Query, column: Expression<V>) -> Expression<Bool> {
+    return infix("IN", column, wrap("", values.selectExpression) as Expression<()>)
+}
 
 // MARK: - Modifying