Using HTML minifer instead of XHTML
Fix Add _references.js command

Command now correctly disappears on other folders, and works on the
project level too.
Add Go to Definition for Javascript <reference>s
Allow app-relative paths in reference go-to-def
Hide _references.js command for non-web projects
Fix relative path completions in CSS URLs

ToAbsoluteFilePath() was being called incorrectly; I fixed the call and
clarified the parameter names
Comments & general cleanup for ProjectHelpers

Mostly tested.