ruby: define 'Migration#id' attribute
ruby: define 'Migration#up_script' and '#down_script' attributes
ruby: change 'Migration#{up,down}_script' to consider '@{up,down}_script' ivars
ruby: change 'DBMS::Base#get_migrations()' to set id into migrations
ruby: define '#_execute_sql_and_get_column_as_text()' in 'DBMS::Base' and subclasses
ruby: define 'DBMS::Base#_get_down_script_of()'
ruby: enhance 'DBMS::Base#unapply_migrations()' to support 2nd flag
ruby: change 'Repository#unapply_migrations()' to support 2nd flag
ruby: enhance to support 'unapply -x' which unapply migration with down-script in database
ruby: update 'README.md'
ruby: update test script
Merge branch 'feature_unapplyforcedly' into ruby-dev
ruby: change 'Migr8::README' to remove extra indentation
ruby: fix 'new --table=name' to set table name correctly