Added PCL library

As I indicated in my issue #1 report, it should be straightforward to create a portable class library for CBOR, and here it is :-)

Since you added `BigInteger` as an explicit member of the library, it was even possible to target more platforms. The PCL can now be consumed by these targets:

* .NET Framework 4 and higher
* Windows Store apps (Windows 8 and higher)
* Windows Phone 7 and higher
* Silverlight 4 and higher
* XBox 360

I have also maintained the original .NET-only project in the solution, and separated the `CBORException` class into two files (using the `partial class` approach). That way I could separate the functionality that was specific to .NET, i.e. serialization, in one file that I only include in the .NET project.

I hope you like this. Hopefully it will enable broader usage of your library.

Best regards,
Anders @ Cureos

PS. Locally, I referenced the portable library in the unit test project, and all tests passed.
PS2. I have used Visual Studio 2010.