Reduce public API methods from NavbarHelper

Before this PR, including `bh` in an app would include more methods
than necessary for navbar: methods like `navbar_id` and
`navbar_string` that should only be accessed privately.

This PR extracts those private methods into a new Navbar class
that includes all the business logic to render elements and edit
attributes (e.g., attach classes), leaving the helper cleaner.
Merge pull request #65 from claudiob/reduce-navbar-overhead

Reduce public API methods from NavbarHelper