Reduce public API methods from CdnHelper

Before this PR, including `bh` in an app would include more methods
than necessary for CDN helpers: methods like `bootstrap_asset` or
`cdn_asset`, that should only be accessed privately.

This PR extracts those private methods into a new Cdn class
that includes all the business logic to render elements and edit
attributes (e.g., attach classes), leaving the helper cleaner.
Merge pull request #53 from claudiob/reduce-cdn-overhead

Reduce public API methods from CdnHelper