and rename documentation links from .html to .md, since
markdown-gradle-plugin will now automatically transfer the links .md to
.html.
Now users can broswe the documentation in their markdown form (e.g. via
github) and via html.
Also add a symlink from README.md to index.md in documentation/.
Introducing a clean split between the constant connection configuration
parameters, which are now all in ConnectionConfiguration and the dynamic
connection state (e.g. hostAddresses) which are now in
AbstractXMPPConnection.
Also removed all arguments of login() since the username, password,
resource and callback handler need now to be configured via
ConnectionConfiguration.
Also remove documentation/extensions/messageevents.md, as it's already
in documentation/legacy
