Re-creating Portable.CBOR as portable class library

Dear Peter,

when reviewing the project, I noticed that the Portable.CBOR project was no longer portable. I took the liberty of re-creating it as a PCL again.

The project file included in the pull request has been created with VS 2013 Update 2, but I have managed to open it in VS 2010. (I am not sure, but if you only have VS 2010 installed you might need to make some updating of the PCL definitions.)

The PCL profile is 136, which in clear text means that the PCL can be used with the following targets:

* .NET Framework 4 and higher
* Windows 8 (f.k.a Metro) and higher
* Windows Phone (Silverlight) 8 and higher
* Silverlight 5
* Xamarin.Android
* Xamarin.iOS

I hope you are OK with this correction, as far as I can tell it should not have any negative impact on the project.

Best regards,
Anders @ Cureos