Throw DuplicateDefintion error if a custom element is registered more than once
add test for duplicate names
Make name argument to document.register case insensitive
Reorder error checking

- Only call .toLowerCase once on definition
remove extra space
Merge pull request #83 from azakus/throw-on-duplicate

Throw DuplicateDefintion error if a custom element is registered more than once
README: note element upgrades happen at DOMContentsLoaded time
depend on 'tools'