Develop an XMPP URI #19

This will be deceptively similar to #15 but with some important differences:

1. XMPP URIs have a resource element, e.g. glyn@example.org/resource
2. XMPP URIs can contain unicode values.

If we follow the scheme described in #18, the headers should be in:

boost/network/uri/xmpp/uri.hpp