Initial, incomplete port of Common & WindowsRuntime code to PCL.
- Changed conditional compilation code to use WINDOWS_RT constant consistently to resolve class visibility conflicts between Common and WindowRuntime code.
- Changed "#if !ASPNET_K" to "#if WINDOWS_RT" to exclude WinRT-specific code from the portable project as well as from the AspNetK project.
- Changed "#if ASPNET_K" instances to "#if ASPNET_K || PORTAB:E" to include most of the K code in the portable project as well.
- Changed "#else" instances to "#elif WINDOWS_RT" to clearly designate WinRT-specific code and make it easier to find later once it can be removed.