definitions for node and edge schema; refs #13
update tests to use minitest; refs #13
bump supported versions for json-schema/rake
refactor; use new minitest api
corrected BEL schema, details follow

- moved evidence inside definitions section
- moved edge inside graph/properties
- fixed evidence JSON pointer in edge metadata
- fixed citation/type string enumeration
- added BEL test to validate child schema and an example BEL network
fixed for BEL child schema; better support

- fixes
  - moved evidence inside definitions section
  - moved edge inside graph/properties
  - fixed evidence JSON pointer in edge metadata
  - fixed citation/type string enumeration
- added test for schema/example validation for bel child schema
- refactored tests to subclass Minitest::Unit::TestCase
- bumped Gemfile.lock to use latest json-schema and rake deps