update tests docs, align with code

[pvgenuchten]

Overview

Update docs to reflect latest code

• use pytest over py.test
• add instructions to install pytest
• fix oarec url
• add code snippets for csw and cite in line with oarec

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines

[tomkralidis]

pip install -r requirements-dev.txt -> pip3 install -r requirements-dev.txt

[pvgenuchten]

PR updated