Drops Liquid processing in assets.

Liquid processing was introduced for the great good (#6), unfortunately it makes
way more harm that good. So dropping it completely. If you need plain CSS (for
example) and asset_path helper inside of it, use ERB:

- rename `filename.css` to `filename.css.erb`
- now you can use `<%= asset_path "my-asset.png" %>`

Resolves #25.
Update HISTORY
Version bump to v0.4.0