Support multiple sessions

[mindplay-dk]

Add support for multiple sessions by adding two optional arguments to createSession():

• $cookie_name defaults to self::COOKIE_NAME, but can be used to optionally specify a cookie-name and create custom sessions.
• $path defaults to "/", and can be used to optionally restrict the cookie to a given path.

For example, a dedicated cookie could be generated for a back-end system restricted to paths under /admin - this would be unreadable from the public pages on the site.