Fixes #125 - Avoid double-prompting on initial login

Previously if prompt=login was passed for the authorize request and the
end user was not currently authenticated with IdSrv3 they would be asked
for a username and password, twice, before being redirected back to the
client.

Now they will only be challenged once, in both the 'user is currently
authenticated' and the 'user is not current authenticated' cases.
Move CoreSettings to Models
WebEssentials
Cleanup usings
Move AurthenticateResut to Authentication namespace
Changed to action results in discovery endpoint
Cleanup config classes
Changed methods to properties in CoreSettings / added additional config settings
Changed IssuerUri to a property
Merge branch 'master' of https://github.com/thinktecture/Thinktecture.IdentityServer.v3
Changed to manual URL creation for form post sample
Added EndpointSettings
Reformatting
Updated EndpointSettings
Add new DataProtection code prototype
Replace old JWT message protection with IDataProtector
Merge pull request #126 from ciaranj/fix_initial_prompt

Fixes #125 - Avoid double-prompting on initial login
Silly renaming exercise
Added license headers
Removed old protection mechanism