Fix some bugs in Yadis discovery

#1 if the xrds doc address was find through header, an https url was
required, that is not necessary.
#2 some xrds docs haven't the priority attributes on the service tag.
Use of async / await
Adds the possibility to skip the discovery phase

Adds the possibility to specify directly the provider login uri instead
of the provider discovery uri.
Merge branch 'master' of https://github.com/laedit/OwinOAuthProviders into laedit-master
Update Nuget package