Standardize License field entries as much as possible

License field should offer different common licensing to chose from and allow adding your own, if necessary. This would make the publications easily sortable by license (e.g. GFDL, CreativeCommons-3.0-CC-BY [creative commons would need a country tag, since they vary between countries or just assume that the country of the publication would be the country of the license as well], private use license, public domain, etc.).