System.Threading.{Semaphore|EventWaitHandle|Mutex}.TryOpenExisiting().
System.Threading.SemaphoreSlim.WaitAsync().
Add System.WeakReference<T>.
Minor .NET 4.5 API fixes.
DataContractSerializer changes for .NET 4.5.

* Move System.Runtime.Serialization.Json from System.ServiceModel.Web.dll
  into System.Runtime.Serialization.dll on the mobile profile.

* Add DataContractJsonSerializerSettings and DataContractSerializerSettings..