3 bugfixes/enhancements following a feedback

Following the feedback of PinpointTownes in a [Katana Project thread](https://katanaproject.codeplex.com/discussions/448615#post1197969) :
 - Fix a bug in Yadis Discovery when the XRDS doc is malformed
 - Proper use of async / await
 - Adds the possibility to skip the discovery phase by indicating directly the provider login uri instead of the provider discovery uri

All of the modifications can be tested via the sample application.
Let me know if you need anything else :)