Initial port of Common, AspNet_K & 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.
- Created Portable.sln to isolate portable work from the rest of the code
  for now
- Created AspNet.shproj, WindowsRuntime.shproj and Common.shproj shared
  projects to reuse ASPNET5, WindowsRuntime and Common code in the
  Portable binary.
- Changed MD5Wrapper and CryptoUtility to use PCLCrypto NuGet package in
  portable version.
Add ASP.NET vNext dev feed to the NuGet configuration so that AspNet_k solution can restore its package dependencies automatically.