Issue 91 : Inject PluginConfiguration from a remote bom

Pull request for #91 
Something to note: unless you inject some arcane xml attributes to control deep merging, individual subsections will be treated as units when deciding which document wins. IIRC, this exact merge method is used when configuring plugins in Maven itself, so the behavior of things like includes/excludes blocks and other lists will be present (and consistent, I suppose) here.

If you want deep merging, then we'll have to either dig up the xml attributes that control it for xpp3dom (a hack I introduced, unfortunately), or we can investigate alternatives.