It's a common pattern to look for all services hosted under the users
service that provide a given feature. Let's provide an API for that and
use that API in the methods that benefit from it.
Re-work filetransfer/bytestream stanza toXML() method to use
XmlStringBuilder. Move the ELEMENT and NAMESPACE definitions in the
right place, ie. the stanza class.
instead of using the old baseName=smack appendix=project.name approach,
we are now going convention over configuration and renaming the
subprojects directories to the proper name.
Having a prefix is actually very helpful, because the resulting
libraries will be named like the subproject. And a core-4.0.0-rc1.jar is
not as explicit about what it actually *is* as a
smack-core-4.0.0-rc1.jar.
SMACK-265
