Add custom serializers to MobileFormatter

[rockfordlhotka]

Closes #2531 Closes #1877 Closes #2148

[rockfordlhotka]

Here's a very simple custom serializer so you can see they work:

https://github.com/MarimerLLC/csla/blob/88d55a96b039df15f9867ab6542c0ee446df5609/Source/Csla.test/Serialization/CustomSerializerTests.cs#L109

You do have me rethinking how a type is mapped to a serializer though. Right now I'm doing it with a direct type comparison, not counting inheritance or interfaces. I suppose I could broaden it so a serializer gets registered for types that can be derived from the type in the map?

Right now:

• MyType => MyTypeSerializer

But maybe:

• IMyType => MyTypeSerializer
• MyTYpe : IMyType=> MyTypeSerializer
• SomeotherType : IMyType => MyTypeSerializer

[rockfordlhotka]

@StefanOssendorf I think this is ready for review if you'd like to take a look.