Made minor cleanup changes.

- 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 Code Sniffer.
Contribution Guidelines and PHPCS Composer Removal

- Made changes to contribution guidelines to include instructions on how
  to use PHPCS.
- Removed PHPCS as a Composer development requirement.
Merge pull request #21 from mdwheele/source-cleanup

Minor Cleanup Changes