Rename Subdomain to Domain Model

[0.19.0] - 2016.12.09 - Rename Subdomain to Domain Model

• 2016.12.09 - Rename package longevity.subdomain to
  longevity.model. Also longevity.exceptions.subdomain to
  longevity.exceptions.model.
• 2016.12.09 - Rename longevity.model.Subdomain to
  DomainModel.
• 2016.12.09 - Rename annotation
  longevity.model.annotations.subdomain to domainModel.
• 2016.12.09 - Move config classes LongevityConfig, BackEnd,
  MongoConfig, CassandraConfig, TestConfig, and
  PersistenceConfig from package longevity.context to new package
  longevity.config.