Make boot-time shadow dom warning a log instead

Change console.warn() on boot when platform.js is not first script on page to console.log() - some javascript testing frameworks insert javascript at the start of the html to assist with accurate testing, and will fail tests if they see console.warn().
Prefix message about not being first script with Warning:
Merge pull request #50 from jerub/master

Make boot-time shadow dom warning a log instead