Initial implementation
Merge branch 'v.next' into dom/issue90_DesktopChallengeHandler
Add support for certificates. Rename SignInChallengeHandler. Clean up.
Merge branch 'v.next' into dom/issue90_DesktopChallengeHandler
Change 'Remember my credential' label
Merge pull request #92 from Esri/dom/issue90_DesktopChallengeHandler

SignInChallengeHandler for Desktop