Fix filenames with multiple dots #90

The correct file extension is not being appended when JavaScript and Stylesheet file names have multiply dots. A quick workaround can be to append the extension `.js` or `.css` to the end of the file name.

It appears that on `line 44` of [renderer.rb](https://github.com/ixti/jekyll-assets/blob/master/lib/jekyll/assets_plugin/renderer.rb#L44) `File.extname(@path).empty?` is returning `false` if there are multiple dots in the file name and therefore not appending the correct extension to JavaScript and Stylesheet files.

To fix I created a method that is used instead of `File.extname(@path).empty?` that will return `true` if the file extension is `.js` or `.css`. Now multiple dots in the file name won't be confused as being the file extension.