Advertising registration as a stream feature

[architecta]

It would be good if jabberd2 advertises registration as a "stream feature". Some XMPP libraries such as the agsXMPP SDK has some problems with registration on jabberd2 as the server doesn't advertise it as a stream feature. This cause the client to think that registration is not allowed on the server though it is.

I tried to force the library to send a "Register IQ" stanza even if the server doesn't advertise registration as a stream feature and it worked. But it would be great if jabberd2 supports that registration stream feature.

[smoku]

http://www.xmpp.org/registrar/stream-features.html

[smoku]

(In [324]) Implemented auth/reg stream features advertisement. Fixes #118