new options: session.cookie persistent and session.check.ssi

Options to enable persistent sessions.

Usage example:

<pre>
function persistent_session(lifetime)
	session.persistent = true
	session.check.ssi = false --ssi will change after browser closes
	session.check.ua = false  --user could upgrade the browser
	session.cookie.lifetime = lifetime or 2 * 365 * 24 * 3600 --2 years
	return assert(session.start())
end
</pre>