fix(ngClass): support multiple classes in key
style(ngClassSpec): fix indentation to make jscs happy
docs(CONTRIBUTING.md): fixing link to Google Closure I18N library

The old seems to link to the source code of I18N. Found the same folder on their new Github repo.

"Closure Library's source repository has moved to GitHub.", https://code.google.com/p/closure-library/

Closes #7638