PROV-913 Relationship generator plugin with tests

+ Relationship generator plugin adds and removes relationships based on configurable rules.
+ Default configuration file (which contains no rules and some sensible defaults).
+ Unit tests for configuration checking functionality of plugin, with (invalid) configuration files.
+ Unit / integration tests for functionality of plugin, with configuration file template that generates
  unique configuration per test run.
+ Deep .gitignore file ensures generated configuration files are never added to VCS.
+ Add tests to (and reformat whitespace in) phpunit.xml.
Merge branch 'PROV-913' of github.com:leftclickben/providence into PROV-913