Support Express 4

- Pass Store base class directly instead of connect
- Remove peerDependency on connect
- Update README example

`express.session` is longer accessible in Express 4 and will throw the following error now when initializing this module with express:

```
Error: Most middleware (like session) is no longer bundled with Express and must be installed separately. Please see https://github.com/senchalabs/connect#middleware.
```

I propose changing what is passed to be the `Store` class directly.  You would probably want at least a minor version bump before publishing, if not a major, because of the change in what is being passed.