Closes #4919

$ curl -s http://www.ietf.org/rfc/rfc4287 | grep -io ATOM | sort | uniq -c
    582 atom
    175 Atom

Adds `image_url`, `javascript_url`, `stylesheet_url`, `audio_url`,
`video_url`, and `font_url` to assets tag helper. These URL helpers will
return the full path to your assets. This is useful when you are going
to reference this asset from external host.

* A `/` is required if you want to explicitly skip the assets directory. This is important because `public/assets` is typically git-ignored.
* The asset id is no longer generated.

American English(according to Weber)

The hostname used in these comments actually exists, which is
undesirable. See also RFC 2606.

Switch to asset_path and make it available in the Sprockets::Context (now you can do asset_path("logo.png") in a stylesheet.css.erb file and get fingerprinting)

ruby 1.9.2 and other ruby implementations may not return the same hash value for the same string each time. This can result in your static assets being served from different asset hosts, which makes browser caching less effective. Use md5 or some other digest method instead.

reorganised the the common asset helpers module into a class and have it include the id caching module, this class is now shared from the view instance to the asset include tag helpers (js and css)

reduced duplication between the javascript and stylesheet asset tag methods, also split the asset id caching methods into a separate module for easy inclusion and use by the asset include tag class and base asset tag helpers

separated the asset id methods to a separate module, removed some dupliation with the various path methods, and moved the base asset tag methods to a base module so the asset id module can play nice with the path generation

split the javascript and stylesheet tag helpers into separate files as a precusor before removing the duplication between the two

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

[#5770 state:resolved]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>