Enhance cache-busting options

I am nudging this now that Rails 3 is out and (hopefully) someone has
time to review it

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/5190-patch-enhance-cache-busting-customization

The current Rails cache busting regime (in ActionView::Helpers) gives
you three options:

* asset_path + mtime query string (The default)
* RAILS_ASSET_ID environment variable used in place of mtime
* A proc called on config.action_controller.asset_path

The proc receives the asset_path, and the user can prepend a directory
name (for example) for each release so that remote caches refetch the
asset:

RELEASE_NUMBER = 12345
config.action_controller.asset_path = proc { |asset_path|
  "/release-#{RELEASE_NUMBER}#{asset_path}"
}

This patch also passes in the asset_id to the proc (using the internal
asset_id caching mechanism), allowing the user to insert the mtime
into the filename:

config.action_controller.asset_path = proc { |asset_path, asset_id|
  asset_path.clone.insert(-(File.extname(asset_path).length+1),"-#{asset_id}")
}

This is the optimal regime for single server deployments (and those
where the mtime can be synchronized between servers) because only
assets that are actually modified will be refetched by the client
browser or intermediate cache. On a site with many static assets and
unique browsers this will reduce massive traffic spikes after a
deployment.

The alternative is for the user to override
ActionView::Helpers.rewrite_asset_path, but they would also have to
handle the different Rails environments.

The submitted approach avoids having to to do this by extending the
current method of altering the asset_path as a config option.

I have added tests and updated the documentation in the helper module.

regards,
Richard