Minor Cleanup Changes

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