remove version info from master branch
dispatch WebComponentsReady event on document instead of body.
Avoid conflicts with using Functions as Custom Element definitions

Function.name is readonly, use defition.elementName to store tagname
Add test
Fixes #362
use two underscores, less likely to have conflict
support document.registerElement
enteredViewCallback -> attachedCallback
leftViewCallback -> detachedCallback
update tests: register -> registerElement; entered/left -> attached/detached.
update tests: register -> registerElement; entered/left -> attached/detached.
README: custom element callback renames
listen for HTMLImportsLoaded only if HTMLImports.ready is false