code: function (db, directory, opts) {
              this.db = db;
              this.directory = directory;
              opts = opts || {};
              this.table = opts.table || this._static.DEFAULT_SCHEMA_TABLE;
              this.column = opts.column || this._static.DEFAULT_SCHEMA_COLUMN;
              this._opts = opts;
          }
docs: Abstract Migrator class. This class should be be instantiated directly.
  
  @constructs
  @param {patio.Database} db the database to migrate
  @param {String} directory directory that the migration files reside in
  @param {Object} [opts={}] optional parameters.
  @param {String} [opts.column] the column in the table that version information should be stored.
  @param {String} [opts.table] the table that version information should be stored.
  @param {Number} [opts.target] the target migration(i.e the migration to migrate up/down to).
  @param {String} [opts.current] the version that the database is currently at if the current version