Provide mechanism for application to store client state

[jkabonick-ft]

Updated the redirect methods for login and logout to accept a client state which can be stored and processed during the callback into the client app.

Process callback params will return the client state object if it exists

[brockallen]

Closing this for the same reason here: https://github.com/IdentityModel/oidc-client-js/pull/16#issuecomment-214892904