Add withTransactionSerializable
Tweak API of withTransactionSerializable

Namely, drop the ReadWriteMode argument, and add a more general
withTransactionModeRetry.
Merge pull request #34 from joeyadams/withTransactionSerializable

Add withTransactionSerializable
Version 0.2.4.0;  CHANGES
Documentation Fix

As promised in github issue #37 and private emails with rekado
Bump version 0.2.4.1; CHANGES