Support express 4.x sessions

Modify the wrapper-function to accept both connect/express object (v3.x)
and the session middleware factory function (express v4.x).

Usage:

```js
// express v3.x
var session = express.session;
var SessionStore = require('strong-cluster-connect-store')(session);

// express v4.x
var session = require('express-session');
var SessionStore = require('strong-cluster-connect-store')(session);

// express v3.x (backwards compatibility)
var SessionStore = require('strong-cluster-connect-store')(express);
```

Inspired by #9.

/to @sam-github please review