Accept other options for Booleans values

Hello,
We noticed (incorrect  but  ) booleans values in JSON returned were failing.
With this change this JSON would accepted:
{
    "testBool": "True",
    "testBool2": "False",
    "testBool3": "T",
    "testBool4": "F",
    "testBool5": "NO",
    "testBool6": "YES"
}

Sources/Unbox.swift

@@ -219,6 +219,12 @@ extension Bool: UnboxableRawType {
     }
 
     public static func transformUnboxedString(unboxedString: String) -> Bool? {
+        for element in [ "true", "t" , "y", "yes" ] where element.uppercased() == unboxedString.uppercased() {
+            return true
+        }
+        for element in [ "false", "f" , "n", "no" ] where element.uppercased() == unboxedString.uppercased() {
+            return false
+        }
         return nil
     }
 }