Updated for Windows 8.1

Updated the 4.5 portable class library to support Windows 8 as well
Updated the WP8 project to reference the portable 4.5 library
Updated the RT project to not have the nuget packages (not needed)
Updated the RT project to reference the 4.5 PCL
The async crash handler call was raising a warning when it didn't need to

Just assigned the response of the async call to a variable to handle the warning message.
Updated readme.md to include Windows RT info
Merge pull request #6 from rmaclean/develop

Added support for Windows 8.1