Move templateContent from window to Platform
Fix StyleLoader to resolve paths to the ownerdocument, not the main document

This fixes use cases that have CSS Imports inside of HTML Imports

Add flag to keep urls absolute when resolving css text through StyleLoader.loadStyles
Polymer will call the API in a way to relativize paths to main document as necessary

Refactor recursive style import test to include use of custom elements, shadowdom, and HTML Imports