Changed filter name and registered filter acccording to documentation

I was having issues getting this plugin to work with the latest version of jekyll (it didn't render or return an error). I noticed that it didn't fit exactly with the documentation so I updated it to follow the same convention and it seemed to work afterwards.

https://github.com/mojombo/jekyll/wiki/Plugins
```ruby
module Jekyll
  module AssetFilter
    def asset_url(input)      
      "http://www.example.com/#{input}?#{Time.now.to_i}"
    end
  end
end

Liquid::Template.register_filter(Jekyll::AssetFilter)
```