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
test for detachedCallback firing without attachCallback
Add multiple insert test for attachedCallback
use same console grouping as inserted
Keep track of inserted/removed for every upgraded element

The code that counts the inserted/removed state must not be gated on callbacks since the attached and detached
callbacks can be set without its the twin, which would break the counting code.

Fixes #89
Rewrite inserted and removed clauses to match old semantics
Merge pull request #90 from azakus/detach-without-attach

Fix inconsistencies with Detach and Attach callbacks when only specifying one of the pair
registering an element after an import has loaded will now upgrade the element inside the import.
Merge remote-tracking branch 'upstream/master'

Updating to latest Custome Elements spec.