Hooktest

- Added required validators package to setup.py
- Declared long description type so that documentation will show up better on PyPI
- Adding parsing of setup.py install_requires and tests_require from the requirements.txt file

- Added auto as a scheme for automatic RNG detection
- This detects the RNG from the <xml-model> element in the XML header
- Downloads the RNG if it is a URL
- Tests the file using this RNG
- Added possibility to give a local RNG file path in the --scheme argument to test against a custom local RNG
- Added --guideline argument in which the version of the CapiTainS guidelines should be specified

- Added unit tests to make sure the empty references and MissingRefsDecl exceptions print in the console

- Added tests for new EmptyRefsDecl exception
- Edited several tests and unit tests to deal with this new exception

- Removed legacy functions
- Cloning repository is not available anymore
- Inline printing is not available anymore, console defaults to table if called

- Implemented test for empty reference strings