[5.2] New methods: SessionStore::increment and SessionStore::decrement

[dereckson]

Several stores in the framework offer increment and decrement
methods as a convenient helper method for get/set.

This change implements such methods in the session store,
to ease the tracking of a state into a session.

For example you can:

        $redirectsCount = Session::increment('auth.redirects');
        if ($redirectsCount < 3) {
            // Retry to authenticate user to an external service.
        }

[dereckson]

This targets 5.2 as it's a minor feature, fully backwards compatible.

[GrahamCampbell]

Thanks. Please revert the file permission change though.

[dereckson]

You're welcome. PR updated to reset file permission change.

[taylorotwell]

Do we want to add a decrement method before people start asking? 😄

[dereckson]

Indeed, other classes have both increment and decrement, I'm updating the PR.