error handling: fixed event name

Currently pipeline listens to 'error' event which never happens:

```javascript
var newPipeline = function(name, res) {
  var pl = Pipeline.create(app.config.id + '-' + name);

  // set timeout for all pipelines.
  pl.timeout = options.timeout;

  pl.on('error', function(err, results) {
    var res = results[0] ? results[0].res : null;
    ...
```

As a result, you will never see error page when static resource not found (timeout instead).
So this pull request changes event name from 'error' to 'end' (and it works then and i see error page + log entry when file not found).

```javascript
  pl.on('end', function(err, results) {
    var res = results[0] ? results[0].res : null;
    ...
```

Plus, i've updated README with some very basic plugin usage instructions.