Allow easy access to events from report object

Calling Report.events() will return a generator of all the events
belonging to the report.


Example of printing all the events for the last report of node 'node.example.com'
```python
db = pypuppetdb.connect(host='puppetdb')
node = db.node('node.example.com')
for report in node.reports():
    for event in report.events():
        print event
    break
```
---
Practical example from puppetboard
```python
   ...
    reports = puppetdb.reports('["=", "certname", "{0}"]'.format(node))

    for report in reports:
        if report.hash_ == report_id:
            events = report.events()
            return render_template(
                'report.html',
                report=report,
                events=yield_or_stop(events))
```


Let me know if I am on the right track :train2: