castle_gates_keys instead of keys table

This is more descriptive and less conflicting with other things such
as the sql keyword or the outdated translation keys table.
fixing schema dumper to use mysql2 and present?

MysqlAdapter will raise a NameError because now we use mysql2
.present? can be used as select(&:present?) and is simpler than
select(|length| length.chars.any?)
dumped schema
change keys table name in sql conditions and README
simplify conditions for holder with new rails 3 syntax
adopt fixtures to use castle_gates_keys table