Default config settings cache and cache_expiry

[mikeytag]

The default config setting for cache is currently set to "private_no_expire" and the setting for cache_expiry is set to 10.

The defaults in PHP are actually "nocache" and 180. This caused an issue for our app that I caught today but I think it makes sense that php-mongo-session's defaults should be the same as PHP out of the box for things like this. I have changed my default config to:

        'cache' => 'nocache',
        'cache_expiry' => 180,//minutes

I recommend that the settings be updated in master to have these important settings (they control the headers passed to clients) set to PHP defaults.

[nicktacular]

That makes sense. Could you make a PR for this?