added method and tests

ExSwift/Array.swift

@@ -1098,9 +1098,19 @@ internal extension Array {
         return sorted(isOrderedBefore)
     }
 
+    /**
+        Sorts the array by the value returned from the block, in ascending order
+
+        :param: block the block to use to sort by
+        :returns: an array sorted by that block, in ascending order
+    */
+    func sortUsing<U:Comparable>(block: ((T) -> U)) -> [T] {
+        return self.sortBy({ block($0.0) < block($0.1) })
+    }
+
     /**
         Removes the last element from self and returns it.
-    
+
         :returns: The removed element
     */
     mutating func pop () -> Element {

ExSwiftTests/ExSwiftArrayTests.swift

@@ -33,6 +33,13 @@ class ExtensionsArrayTests: XCTestCase {
         // check that the destination has been sorted
         XCTAssertEqual(sortedArray, [2, 3, 5, 6])
     }
+
+    func testSortUsing() {
+        let sourceArray: [Int] = [3, 2, 7]
+        XCTAssertEqual(sourceArray.sortUsing({ $0 / 3 }), [2, 3, 7])
+        XCTAssertEqual(sourceArray.sortUsing({ $0 % 3 }), [3, 7, 2])
+        XCTAssertEqual(sourceArray.sortUsing({ -$0 }), [7, 3, 2])
+    }
 
     func testReject () {
         var odd = array.reject({