Adopt richness of information metric to tire patch

After committing a pull request to fix the functionality of tire's
to_hash method I have also changed the code for the richness of
information metric implementation.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add metadata number property for repositories

Just added, respectively renamed, the property for holding the number of
metadata a repository has.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Include notes field into RoI metric

The notes field is another CKAN core field with descriptive data. It is
as important as the resource description fields. Hence, I have added it
to the richness of information metric implementation.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>