Export the pouch so apps that embed express-pouchdb can access this pouch.

I have just tried a few different ways of exposing the Pouch instance. This above is successful. Here is how you might use it in an app:


```
var express = require('express'),
    express_pouchdb  = require('express-pouchdb'),
    Pouch = express_pouchdb.Pouch,
    port    = 15984,
    app     = express();

app.use('/_db', express_pouchdb);
app.use(express.static(__dirname + '/public'));
app.listen(port);



Pouch('leveldb://firstPouchapp' , function (err, db) {
    if (err) return console.error('cant init db');
    db.post({name: 'Ryan'}, function(err){

    });
});

```


I have tried the following options, which all failed:

  - in express-pouchdb, add app.set('pouchdb', Pouch) but app.get('pouchdb') was unset.
  - Add a pouchdb dependency to the app project, and simply require('pouchdb'), but this creates the data files in node_modules/pouchdb/tmp/data and the express app stores the files in the root dir
  - require('node_modules/express-pouchdb/node_modules/pouchdb') but like above this stores the files in that folder.


Anyway, the export seems a sane a clean approach.
Merge pull request #4 from ryanramage/patch-1

Export the pouch so apps that embed express-pouchdb can access it