Add support for Visual Studio's Ref12 Extension

[JoshVarty]

@SLaks has created a Visual Studio Extension called Ref12.

It allows users to use F12 (Go to definition) on .Net and Roslyn types and methods.

He suggested we could create an HTTP API that exposed the assemblies for which we had source code. This would allow him to add support for Source Browser assemblies within his extension.

We will need to do two things:

1. Expose a list of assemblies like the .Net Reference source does. See: http://referencesource.microsoft.com/assemblies.txt
2. Map between symbol IDs and the correct page and line number.

[SLaks]

Further details:
1 would be a simple HTTP API.

2 would be a URL that Ref12 would navigate users to, and would redirect users to the actual page with source.
Ref12 needs to be able to construct it from only the information that it has available (even from pre-Roslyn editors); this is the SymbolInfo class. I can add more properties to it if you want, but only if they can be constructed from all four resolvers (Roslyn ISymbol, legacy RQName, and legacy VB SyntaxNode; see my source).
It also needs to be a stable URL.