Added Windows Store and .NET 3.5 compatibility

+ Added explicit vProto.Request.SendSynchronous method for sending
requests synchronously;
+ Added support for .NET 3.5 - the only feature missing for now is the
optional vProto.RMI.BaseService class;
+ Compiles for Windows Store as a class library, but features no
client/server yet;
> RMI client proxies dictionary now stores nullable key-value pairs
instead of tuples.

It was dirty, but doable. So I did it.
Usable clients and servers will be added shortly for Windows Store.
Windows Phone and Mono support is also a target for the future.