Remove check if server advertises In-Band registration as stream feature.

According to http://xmpp.org/extensions/xep-0077.html#streamfeature,
the server MAY announce support for In-Band-Registration.
Add support for x-data-forms (XEP-0004) in In-Band-Registration.

The support is basic, only fields of type 'text-private', 'text-single'
and 'hidden' are supported.