Issue that category name use Chinese character

if category name uses Chinese character, it will display the Chinese character directly in the url . 
After all it should use the same ```to_url()``` function that's used in [category_generate.rb](https://github.com/imathis/octopress/blob/master/plugins/category_generator.rb#L176) in the first place.
Merge pull request #1 from francis-liberty/patch-1

Issue that category name use Chinese character