Multi domain support

Multi domain support (virtual hosts) for Perl Ocean.

With this the following becomes possible:
- accept multiple domains in the same ocean cluster
- separate (or not) the presence/roster based on the domain
- allow/disallow message between domains

The domain is determined the following way:

- xmpp
the domain received from the client at the beginning of the <stream>

- httpbinding
the HTTP_HOST environment variable

It is then passed to the backend handlers on authentication and binding requests. Samples can be seen on the built-in handlers.

Modification to the configuration file is required. The frontend domain configuration now accepts a list of domains instead of just one domain and the backend configuration doesn't accept domain specification anymore as it is passed to the handlers.