Add EntityID

[miketonks]

We would like to be able to specify the EntityID in the Service Provider, instead of using the Metadata URL.

I found a PR for same issue, although it was closed without comment. #239

I will make a new PR based on this, as I see a few things have changed so it needs updating.

[crewjam]

Sounds good to me. I'm not sure why the author closed it, but it seems like an appropriate change. Thanks!

[miketonks]

Submitted PR: #258

I've handled the fallback to MetadataURL in multiple places, as that seemed the best way to ensure backwards compatibility and not break existing tests.

I've also added a new test for EntityID based on TestCanProduceMetadataNoSigningKey.

Note: this is working for me with Azure AD.

[miketonks]

Thanks for accepting the PR, and responding so quickly :)