Wrap updates in transaction to avoid one BEGIN/COMMIT per row

[JasonBarnabe]

Given n rows processed in m batches, currently 3n statements are sent to the DB for updates: BEGIN, UPDATE, COMMIT. If the updates were wrapped in a transaction, then it would only send n + 2m updates.

On a local postgres table with 40000 rows, batch size 1000, anonymizing a single email field.

Before changes: 2m 52s
With transactions: 2m 26s (15% faster)

I'm not sure if this would have undesired effects for others, so maybe this should be configurable?

[coveralls]

Coverage decreased (-2.3%) to 91.541% when pulling d4f1c30 on kickbooster:transactions into db4f509 on sunitparekh:master.

[JasonBarnabe]

Note that #57 avoids transactions altogether which brings the number of statements to n.