Added the 'numbering' option in the configuration for unordered publicat...

I thought it would be useful to have a configuration option for jekyll-scholar so that the publication list is not numbered. 
I modified the 'bibliography.rb' file so that, if the _config.yml file for the jekyll site has an option **numbering : false**, then the list will be unordered (*ul*), otherwise it will be the standard ordered (*ol*) list.

![_config_yml](https://cloud.githubusercontent.com/assets/430881/4811165/81d87362-5eb8-11e4-88ba-b710c16886e4.png)