Add information desk type (ref #1192)

* [x] Add field in database
* [x] Add some fixtures
* [x] Migration of existing data
* [x] Move to tourism app [cf. procedure](http://stackoverflow.com/questions/1258130/how-do-i-migrate-a-model-out-of-one-django-app-and-into-a-new-one)
* [x] Update REST API of information desk
* [x] Show pictogram in maps in trek detail page 
* [x] Update README