Alternative URN model

(we should fix #84 before this)

Discussion about alternative URN model: there are 3 problems about this model:

1. the field description should not be mandatory. An alternative URN could exist without a description. We should desactivate the link between URN and description;
2. the field keyword should not be mandatory. A alternative URN could exist without a keyword. We should desactivate the link between URN and keywords;
3. we should discuss the relationship between the fields source and URN: 2 options:
   1. we delete the field source and we put just an URN in the field URN
   2. we create a model URN source in which we give a title to the source (for example: Perseus) and the route of the URNs (for example http://data.perseus.org). The rest of the URN will be in the URL field but should be a text field and not a URL field
   3. in source there is just the name of the source and in the URN field we have the complete URL.