Add support for SerializeAsAttribute

[knippers]

Adding support for SerializeAsAttribute (used in the XmlSerializer). Up for debate would be to move the SimpleJson.cs class to Serializers folder or move the SerializeAsAttribute.cs from the Serializers folder one level up.

[johnsheehan]

Thanks for your patch. Should probably discuss this in the Google Group first: http://groups.google.com/group/restsharp

First bit of feedback to launch that discussion: this needs some tests. Also should discuss whether or not changes to SimpleJson are a good thing or bad thing or if it should just support SimpleJson's existing support for attributes.