Net Framework 4.8 with updated dependencies

Solution has been upgraded to .Net Framework 4.8, IdentityModel 4, Microsoft.IdentityModel 5.6 and Micorosft.OWIN 4.1.

You can now also pass in an optional TokenValidationParameters object to the OWIN Startup. We needed this so we could change the NameClaimType and RoleClaimType parameters for Microsoft.Identity, so that the name and roles was set properly on the Identity object.

Automatic build has also been updated to support .Net Framework 4.8