Template based auto-creating Multiple Session Pooling Repository

(1) Make the MultipleRepository be able to create
BasicPoolingRepository automatically by an option.
By default, it remains to be injected with predefined
BasicPoolingRepositories though.
(2) Let's have HST-2 extended credentials format for usernames.
For example, if there are 500 sites, site001~site500, then the
usernames can be used like these: admin@site001 ~ admin@site500.
The part after '@' is just distinguishable place holder in
MultipleRepository; it can separate pools because the strings are
different now! The HST-2 pooling repository should remove the part
during logging in to the hippo repository.
I'm not sure if this is critically breaking the jcr
specification, but it's very intuitive extension, I believe. For
example, in the URI specification, it allows
'http://username:password@www.site.nl/news/' format. (The authority
part.)