Changes to make Symplified.Auth work with ADFS IdP

Using SamlDeflate encoding for SAML AuthnRequest so that integration
with ADFS works as expected.
Also fixed CheckValid for Saml20Assertion to use UtcNow because it
would fail with “Assertion no longer valid”.
Some changes so that OIOSAML.NET source code compiles with Xamarin.

Contribution made under the Apache License, Version 2.0
http://www.apache.org/licenses/