Make it possible to load either rexml or nokogiri

[SchorppDA]

Make it possible to load either rexml or nokogiri.
Since nokogiri is preferably selected in the code, it should also be possible to work without rexml at all.
This is necessary for Ruby installations that don't have the bundled gems, e.g. when used on embedded devices.

[coveralls]

Coverage: 95.279% (-0.05%) from 95.324% when pulling 3e0443e on SchorppDA:master into 5ebbafd on mvidner:master.

[mvidner]

Thanks!

Ideally I would like to say in gemspec that we depend on either rexml or nokogiri but I think the gem system cannot express that (unlike RPM where you can Provide a xml_library symbol to be Required downstream)

I just need to add a NEWS item and comment the requirements in the gemfile.

(Another level would be making a XML parser completely optional, by showing the users how to declare proxies without calling Introspect, and using that in Connection#request_name)