Export Couch constructor.

Exporting the `Couch` constructor function allows more flexibility in the usage of the module. For example, promisifying the `Couch` prototype with [Bluebird](https://github.com/petkaantonov/bluebird):

```javascript
var couch = require('couch'),
    Promise = require('bluebird');
Promise.promisifyAll(couch.Couch.prototype);

var c = couch('http://me.iriscouch.com/db');

c.post({msg: 'new document'}).then(function (info) {
  return c.post({'msg':'new document', _id:info.id, _rev:info.rev});
}).then(function (info) {
  return c.get(info.id);
}).then(function (doc) {
  console.log(doc); // {'msg':'new document', _id:<id>, _rev:<rev>}
}).catch(function (e) {
  // All errors bubble here
});
```