creates system, package & vulnerability models

app/models/package.rb

app/models/system.rb

app/models/vulnerability.rb

db/migrate/20160130061551_create_systems.rb

db/migrate/20160130061647_create_packages.rb

db/migrate/20160130061938_create_vulnerabilities.rb

spec/factories/packages.rb

spec/factories/systems.rb

spec/factories/vulnerabilities.rb

spec/models/package_spec.rb

spec/models/system_spec.rb

spec/models/vulnerability_spec.rb