Added Profile78 v4.5 portable class library

Hi Michael,

Thanks for the library, please see the attached pull request that adds PCL
support which allows the library to run on Xamarin (iPhone and Android) and
Windows Phone.

It will also allow you to start to migrate away from maintaining one version
per .NET framework - reducing your personal support/maintenance costs.

I am seeking to get this integrated upstream into your NuGet feed so that I
don't have to create my own fork, etc. If you have any questions or you need
this cleaned up before integration please reply back.

Regards, 
Geoff

Implementation Notes:
- SystemClock.cs changed to Task.Delay via IFDEFs on PORTABLE
- BrokenCircuitException on PORTABLE is not Serializable.