PROV-913 Relationship generator plugin

+ Plugin which automatically adds and removes relationships based on configured rules.
+ Base configuration for plugin, which does not contain any rules, but has some sensible defaults for other config items.
+ Unit tests for plugin configuration check, with various configuration files that are invalid in different ways.
+ Unit tests / integration tests for plugin functionality, which processes a template into a unique configuration file, and
  exercises the plugin through real database interactions.
+ Low level .gitignore ensures that the generated configuration file is never added to VCS.
+ Add unit tests to phpunit.xml.
+ Reformat whitespace in phpunit.xml.