If the :storage_engine setting is not present, use a default ENGINE
setting of MyISAM instead of taking the default storage engine setting
used by the MySQL server. MyISAM is used as the default because
InnoDB needs an extra operation to get many uids at once.

If the :storage_engine setting is InnoDB, do an
 ALTER TABLE table_name AUTO_INCREMENT = 1
query after the UPDATE query in get_many_uids_for_class, to force
InnoDB to reset its internal auto_increment counter for the table.
Update activerecord dependency to version 3.2.16
Update unit tests to check if MyISAM storage engine is used
when no storage_engine setting is provided, and check if
an interleaved sequence of obtaining single and multiple UID's
returns correct results with MyISAM and InnoDB engines.

Removed some mocha stubs that were throwing exceptions and
didn't appear to be doing anything.
Tighten up the engine_stmt based on @grosser's suggestion
Merge pull request #15 from zendesk/ggrossman/default_engine_myisam

When no :storage_engine setting is provided, use MyISAM; reset MySQL's auto increment counter when InnoDB is used