Add CopyArtifacts target to Build.proj and add nuspecs to /nuspecs

- Automatically pick up new projects in Build.proj
- Add a package-src directory to artifacts
- Group artifacts by target platform
- Publish nuspecs to artifacts
- Publish FxCop logs to artifacts
- Publish jquery.signalR.js and jquery.signalR.min.js to artifacts
- Build Asp.Net samples from Build.proj and publish to artifacts
- Publish artifacts grouped by project to artifacts/projects

This is meant to make signing and localization simpler

#1896 #1897