Asset path helpers for Sprockets 4.x & 3.x & <= 2.2 applications

Sprockets::Helpers adds the asset_path helpers, familiar to Rails
developers, to Sprockets 2.x assets and applications.

Features

* Includes helpers for image, javascript, stylesheet, font, video, &
  audio assets.
* Automatically appends extension if necessary.
* Optionally outputs digest paths.
* Falls back to file paths in the public directory & adds cache
  busting timestamp.