started hacking on custom elements

Everything was going well until I compiled to js and realized that
Document.register is still behind the 'experimental web platform'
chrome:flag in Canary.  I don't want to move to this until I have
assurance I can use it in production Chrome code, which might not be for
a while.  I'm committing this to a branch so I don't lose the code.
pubspec : add `custom_element` to `dev_dependencies` and include the polyfill in showcase example

this solves the Chrome issue :D
ToggleOpen : bug fix in onToggle listener

the local isDown was masking the field isDown which is set by the
document onClick listener; use `down` for the local name to entirely
disambiguate
tags.dart : add `registerTagsCustomElements()` function
tags.dart : guard against double registration in `registerTagsCustomElements`
dartdoc the `registerTagsCustomElements` function
showcase.dart : use `registerTagsCustomElements` function instead of manual registration

set a good example
Merge pull request #5 from rmsmith/custom_elements

Custom elements