This is a helper class that can be used to obtain an IContextId in the following ways:
1) From a local file (an XRDS document)
2) From a XRI (which yields an XRDS document, using XRI resolution)
3) From a URL (which yields an XRDS document, using Yadis discovery)
4) From a stream (from which an XRDS document is read)
5) From the ContextId list that was passed to IdASRegistry using the Configuration API.