fix(chrome installers): Check for existence

 - Don't launch Chrome installers if they're already
    installed
 - Heuristic for Canary is a path check
 - Heuristic for dev channel is path check + reg key
chore(GoogleChrome.Canary): publish 28.0.1461.0
chore(GoogleChrome.Dev): release 27.0.1453.12
feat(chrome extensions: JS Runtime, PrettyPrint

 - Update README
 - Keep an eye on, but don't install CSS Shack / JetBrains IDE
chore(ChromeCanaryDevExtensions): release 0.0.2
chore(ChromeDevExtensions): release 0.0.2
fix(ChromeDevExtensions): nuspec filename