Add JSON dependencies and fixed metrics namespace

For implementing the completeness metric I have added the JSON
dependency to the Gemfile. For the CKAN metadatum I have added
the JSON schema file.

Also I have fixed the metrics namespace for the completeness metric
class. If subfolders are created, apparently one needs to put the
classes inside these subfolders into modules with the same name.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Implement compute completeness metric request

I have implemented the request for computing this metric in a very
simplistic fashion. The database is queried and the results are then
passed into the redirect.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Implement weighted completeness metric

With no difference to the previous metric implementation I have added
the weighted completeness metric. The current code involves too much
redundancy which will be eliminated soon.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Implement richness of information metric

The third metric to be implemented is richness of information as this is
the only metric left which can be implemented without much additional
effort.

The next step will be to rethink the design of the metrics, the data
input etc. When I have figured this out I will probably write more
extensive tests to cover this very functional part of the platform.

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