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).

Modify unit-tests to use express 4.x and standalone middleware packages.

Usage:

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);
Improve jsdoc comments, add example usage