2.1.0

• added support for Postgres databases;
• all export() and import() methods have been moved to the Driver class;
• added afterExport(), afterImport(), beforeExport() and beforeImport
  methods to the Driver class;
• getCompression() and getExtension() moved from Driver to BackupTrait
  class, because these methods are not strictly related to the database engine
  you are using;
• removed getValidExtensions() and getValidCompressions() methods from
  Driver class, because extensions and compressions are the same for any
  database engine;
• removed getDefaultExtension() method from Driver class, because the
  default extension is the same for any database engine.