[#1] First draft of the rdflib-based DCAT parser

Right now it extracts (almost) all fields in the DCAT-AP Swedish
recommendation except for publishers and contact points and returns a
CKAN dataset dict. Whenever possible fields are mapped to existing CKAN
fields, and the rest are stored as extras prefixed with `dcat_`.

The module can be called from the command line for debugging, eg

cat dcat.rdf | python ckanext/sweden/dcat/parsers.py | python
-mjson.tool

or

python ckanext/sweden/dcat/parsers.py catalog.rdf | python -mjson.tool
[#1] Move parser to class so it can be extended
[#1] Store lists as json dumps
[#1] First go at the RDF based DCAT harvester

Inspired by the existing ckanext-dcat harvester, but rather than relying
on parsing text files it uses rdflib to treat them as RDF graphs. It
uses the parser on parsers.py.

The main difference is that all CKAN datasets are generated on the
gather stage, as we have one single graph. The harvest objects store the
json dump of the ckan dicts, which are later created, udpated (or
deleted) in the import stage. This is done so all the reporting,
indexing, etc works as expected.
[#1] Add requirements file
[#1] Add harvester plugin to setup.py
[#1] Improve the CLI for the RDF parser
[#1] Add docstrings, better function names
[#1] Use license spelling to match core fields
[#1] Tweaks in parsers

Added docstrings, renamed DCATAP parser to reflect that is based on the
European DCAT-AP.

Allow parse() to not receive any content.
[#1] Add tests for parsers

Added some examples to be used. dataset.rdf is used to test all fields.
Still some doubts around some of the fields.
Include requirements.txt on travis
[#1] Add a turtle test and more examples
[#1] Fix harvested datasets indexing

Reorder the setting of package_id and current on harvest_object so they
happen before the dataset creation
Add requirements to travis for dcat tests
Run the ckanext-sweden tests only
[#1] Fix deletions
[#1] Add harvesting tests

They require redis to be installed, as they will use the actual queues.
Also the harvest plugins need to be loaded and the --ckan option needs
to be used when running the tests.
[#1] Move dcat requirements to own folder
[#1] Update README with dcat harvesting instructions, and tests