add new register() function to establish zope listener via SQLAlchemy events

this pullreq makes available a register() function which connects the ZopeTransactionExtension instance to a target Session class, factory or instance via direct use of the SQLAlchemy event API available as of SQLAlchemy 0.7.  While SQLAlchemy's SessionExtension is remaining indefinitely, this calling style is the more up-to-date style and also allows for more flexibility as far as the event target, including ad-hoc registration (and de-registration in 0.9) of specific events.