initial commit

- simple data model implemented
- first tests introduced for basic validation of parsed data
refactored implementation of models

- Actor and Repo have been renamed to User and Repository to support the more
  detailed data type that can be obtained from the GitHub API
- Moved constructors above Struct definition
too tired to continue... horrendous neo4j api
refactored package structure to decouple mining and persisting logic
introduced enumeration for node class names
updated test suite to reflect new package arrangement
refactored enumerated types