Fix typo in spec description
add allow_missing_migration_files as option for migrations
only iterate over integer-based migrations to check for existence if allow_missing_migration_files is false
Support :keep_reference=>false Database option for not adding reference to Sequel::DATABASES

This is designed for libraries that use Sequel to connect to
databases themselves.  You don't want the database's they create
to be in Sequel::DATABASES since that could cause them to be
chosen as the default Sequel::Model database.
Fix race condition related to prepared_sql for newly prepared statements (Fixes #651)

Previously, the code was racy because the prepared statement SQL
was not finalized before prepare returned.  If two separate
threads then tried to prepare the SQL for the same time statement
and the SQL hadn't been prepared before, the code could break.

Fix this by making sure that the prepared statement SQL is
finalized before prepare returns.  Note that this only needs to
be done if prepare is given a prepared statement name.  If the
name is nil, it is being called by call, which doesn't need to
prepare the SQL first, since it will be working on a thread-local
clone of the dataset.
Fix example code in serialization plugin (Fixes #653)
Minor tweaks to the allow_missing_migration_files commit

It doesn't make sense to add this to SimpleMigration, since it
is a migrator setting, not a migration one.

Add documentation for the migrator option, fixing existing
RDoc format while there.