#43 — subscriber id calculation & use

don't assume all subscribers emails have been converted to id the same way so users can adapt the conversion method

reduce the risk of having email collision by treating ".", "-", "_", "@" differently than others signs

by the way, in registration.py, the test :
"if subscriber in newsletter_container.objectIds():" is broken because the email hasn't been converted to id

for implementation example, see getNewSubscriberId() and getSubscriberInfo() in bpi-rouet-2011-09 branch