(#204) - Follow db updates

`nano.follow_updates([params], [callback])`

uses follow to create a solid `_db_updates` feed. please consult follow
documentation for more information as this is a very complete api on it's own

```js
var feed = nano.follow_updates({since: "now"});
feed.on('change', function (change) {
  console.log("change: ", change);
});
feed.follow();
process.nextTick(function () {
  nano.db.create('alice');
});
```
Merge pull request #205 from jo/204-follow-db-updates

(#204) - Follow db updates