Improve assets fetching algorithm

Current way of fetching CSS doesn't seem good enough.

When there the url is given explicitly it should not append a digest.
That's why `http://example.com/assets/base.css` should not be changed to
`http://example.com/assets/base-089e35bd5d84297b8d31ad552e433275.css`
(related test deleted). As asset with digest might not exist.

There should be an ability to load css from different host
url than the one in asset_host in rails config.

Also asset_host could be specified without protocol:
`//example.com`.
http://stackoverflow.com/questions/9161769/url-without-httphttps
This case should be supported when fetching asset with http