No results / no entires message

When a group organisation or user has no datasets we should display a message such as:
"This group has no datasets"
or  "This group has no public datasets"
http://master.ckan.org/group/itestgroup

![Screen Shot 2013-02-26 at 10 55 34](https://f.cloud.github.com/assets/1260289/195917/18b1cac2-8003-11e2-9fe8-6f7643c6e3e7.png)