Code Review: Fix Silverlight builds

http://github.com/jschementi/ironruby/commit/e806c00a47e8620385defa348dd702fa35b91430

Add required Silverlight 3 and 4 runtime and SDK binaries so
Silverlight builds can occur without having those Silverlight versions
installed.

  • Adds a SilverlightSdkPath MSBuild variable to Common.proj

  • Adds Clean.bat for cleaning all binaries

  • Updates package_slvx.bat so Silverlight builds work out of the Git
    repo

  • Updates Alias.txt for Silverlight builds

Awesome work, Jimmy :slight_smile:

-Charles