license/licenses revision

Let me know what you feel about this. I feel that we should do more to encourage consistency between the license of the package and the actual license properties, and I hope this language accomplishes that (I'd otherwise prefer to make it even stronger to encourage automatic data processing).

The second part is for the licenses property where it didn't feel sensible to require a url since the license property doesn't require it. Instead, it felt proper to make it optional, but to require it strictly if specifying a license which is *not* an od license.