bugzilla/config.json: remove optional options

Write sane defaults to the platform and the op_sys fields as Bugzilla
demands their presence despite of their optional nature described in
the docs.
bugzilla/xmlbuilder.coffee: escape XML for strings
bugzilla/{bugzillarpc,index,xmlrpc}: don't throw errors

Use the proper way instead. Thanks @ConradIrwin for guidance :)
bugzilla/{bugzilla,xmlrpc}.coffee: refactor XmlRpc object

It turns out Node.js caches[1] module.export objects.

http://nodejs.org/api/modules.html#modules_caching