Minor Cleanup Changes

[mdwheele]

Hello!

In preparing to begin implementation of the discussion in #19 (issue), I made a few small cleanup-related changes and also added PHP CodeSniffer to the Composer development requirements to have a way to scan for compliance after contributing.

• Removed unused import in XmlFileLoaderTest.
• Added default array assignment to AbstractNode's output arcs to match input arcs.
• Added PHP CodeSniffer as a development requirement to help maintain PSR-2 compliance.
• Added documentation to CONTRIBUTING.md for using PHP CodeSniffer

[mdwheele]

If good to merge, let me know and I'll squash commits if necessary. Thanks!

[florianv]

Thanks