Add `:id => true` option to Groonga::Table::KeySupport#delete

GitHub #23:

    `Groonga::Table::KeySupport#delete` accepts both ID and
    key. If passed value is integer, it is handled as ID.
    But we can use `Int32` as key. In the case, we can't
    delete a record by key.

    Add `Groonga::Table::KeySupport#delete(id, :id => true)`
    API. It introduces a backward incompatibility. But it is
    OK because the current API is a bug.