[#75] Schema.org mapping

[metaodi]

Add schema.org mapping (see #75 for details)

This PR provides:

• Mapping from DCAT to schema.org/Dataset
• New parameter profiles to the catalog and dataset endpoints to be able to change the profiles when requesting the metadata (this allows to server DCAT and schema.org on the same instance), example /dataset/my-dataset.jsonld?profiles=schemaorg

[metaodi]

I found a way to check if a helper function is defined.

In https://github.com/ckan/ckan/pull/3042 an excpetion was added to the helper dictionary, so it's no longer possible to simply use {% if h.structured_data %} in jinja, as this triggers an exception. But calling h.get() seems to do the trick:

{% if helper_dict.get('structured_data') %}
{{ h.structured_data()|safe }}
{% endif %}

[metaodi]

Unfortunately the approach I described above does not work in older versions of CKAN (<2.6), because earlier versions of jinja do not support the use of dict(). Therefore I added another template helper to check if a helper is available or not.

[metaodi]

I rebased this PR to master to fix the merge conflicts.

[metaodi]

@amercader ping, can you review this PR?

[amercader]

sorry @metaodi, I'm a bit swamped right now but it's definitely on my list. Will try to review it this week.

[amercader]

This looks good @metaodi . Only one minor change in the README and we can merge.

[metaodi]

@amercader I made the changes to the README and added the new config option for the RDF endpoints (+tests)

[amercader]

Thanks @metaodi, fantastic work as usual

[amercader]

This is worth a new release, I've released 0.0.7