Add redirectHost parameter to OpenID providers.

Allow a web service to specify the hostname used for redirects in the
OpenID protocol. This allows the protocol to work when the web server is
reverse-proxied, for example.
OpenID demo and server support Mongo user & pass.

Updated these with regard to recent changes in the NewMongoStore
declaration to support specifying a MongoDB username & password for the
connection.
Merge pull request #14 from cmars/master

Parameter for redirect host w/OpenID, fixed NewMongoStore calls