Add support for parsing remote IdP metadata about omniauth-saml

[bufferoverflow]

I was not aware of the remote metadata feature, it would make setup much easier. I'm not aware of someone working on this, feel free to provide a Patch, see CONTRIBUTING.md.

[sanderhahn]

At the moment i am parsing metadata and then extracting the Settings properties back into env["omniauth.strategy"].options. Configuration would be nicer if its possible to directly assign a settings property with OneLogin::RubySaml::Settings or a callback for retrieving the settings.

[rhoerbe]

A nice feature of the configuration-by-metadata is the option to do local idp-discovery, particularly if the IDP metadata has mdui_extensions provided.

[peterwake]

If anyone is looking to parse Microsoft federationmetadata.xml, I have written a gist that is quite rough and ready but gets you the key data you will need.. https://gist.github.com/peterwake/18349642f8f16397d1812461bac11aff