Merge pull request #130 from rwjblue/htmlbars-html-validation

Ensure closing tags are present.
Add HTMLBars compatible helpers.

This completely duplicates the internal helpers and conditionally
returns the correct helper (after checking if HTMLBars is enabled).

It is absolutely possible to reduced duplication in these helpers, and I
can try to refactor them if that is required.
Update select-ember-version to also enable HTMLBars.