Doloop

* check() and stats() no longer explicitly lock tables (Issue 22)

* stats() does less, but avoids scanning all rows

* explicitly lock loop tables rather than trusting MySQL
* no retry on deadlocks or lock wait timeouts (no longer needed)
* allow storage engines other than InnoDB

* min_update_time and max_update_time are no longer reversed (Issue 12)

* Updates to stats()
* Cleaner output (Issue 4)
* We now collect update-time stats for locked/bumped IDs (Issue 5)
* No delay thresholds by default (Issue 13)
* There is now a 'total' stat
* should now work with any DBI-compliant MySQL driver (Issue 12)
* tested MySQLdb, pymysql, oursql, mysql.connector
* update()'s return value was useless before; now it's official.
* read-only mode for testing/dry runs (Issue 9)
* breaking ties in get() (e.g. for new IDs) is now left to the database