Implement missing stringValue and descriptionWithLocale APIs

Author: seabaylea

Foundation/NSDecimalNumber.swift

@@ -55,7 +55,7 @@ public class NSDecimalNumber : NSNumber {
         NSRequiresConcreteImplementation()
     }
 
-    public func descriptionWithLocale(locale: AnyObject?) -> String { NSUnimplemented() }
+    public override func descriptionWithLocale(locale: AnyObject?) -> String { NSUnimplemented() }
 
     // TODO: "declarations from extensions cannot be overridden yet"
     // Although it's not clear we actually need to redeclare this here when the extension adds it to the superclass of this class

Foundation/NSNumber.swift

@@ -434,6 +434,10 @@ public class NSNumber : NSValue {
         return val
     }
 
+    public var stringValue: String {
+        return descriptionWithLocale(nil)
+    }
+    
     /// Create an instance initialized to `value`.
     public required convenience init(integerLiteral value: Int) {
         self.init(integer: value)
@@ -452,6 +456,12 @@ public class NSNumber : NSValue {
     public func compare(otherNumber: NSNumber) -> NSComparisonResult {
         return ._fromCF(CFNumberCompare(_cfObject, otherNumber._cfObject, nil))
     }
+
+    public func descriptionWithLocale(locale: AnyObject?) -> String {
+        guard let aLocale = locale else { return description }
+        let formatter = CFNumberFormatterCreate(nil, (aLocale as! NSLocale)._cfObject, kCFNumberFormatterDecimalStyle)
+        return CFNumberFormatterCreateStringWithNumber(nil, formatter, self._cfObject)._swiftObject
+    }
 
     override public var _cfTypeID: CFTypeID {
         return CFNumberGetTypeID()