Add error & completion filter to kill import-once

Fixes #62

The ErrorTagProvider doesn't work because
Microsoft.Less.Editor.ImportDirectiveChecker
swallows all @import-once directives to supress
native CSS errors.
Fixed compiling issues and enabled last features
Removed redundant functionality

This feature is built in to VS2013 RTM
Fixed issue with CSS classifiers
Fixed rules.length null error

Some Chrome Extension add style sheets to the page and the rules can't
be access or return null.
Updated packages. Version 1.0
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