Recursively resolve @import rules

add tests for style loader

Use cssRules, wait for polymer to fully load

Fix bug after removing index as an argument

Add more @imports to test with

Make the XHR loader more generic, expose on Platform.loader.fetch

Split apart "Loading" from "Flattening"

Generic Loader takes urls, returns a map of url -> text.
StyleResolver takes maps of urls and flattens the @imports

Place recursive test in a non-conflicting location