Add 'assets_tag', 'javascript_tag' and 'stylesheet_tag' helper.

'assets_tag' receives a block to construct an arbitrary tag. 'javascript_tag' and 'stylesheet_tag' have corresponding tags predefined.

With options[:expand], all assets required by the asset given as source generate a tag, with body=1 appended to the uri. This option can be toggled globally.
Merge pull request #11 from trkoch/master

tag helpers with optional expanding