Add an ES6Transpile middleware.

This uses the mtime of the files in `packages_es6/` to determine if a
re-transpile is required. If no files exist in `packages_es6/` or the
`bin/transpile-packages.js` doesn't exist this is a no-op.
Merge pull request #129 from rjackson/add-transpile-step

Add an ES6Transpile middleware.