[#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