Add support for filtering on event types to options

Would be nice to be able to specify to only show certain types of events (e.g. commits and issues)

```
  $('#github-activity').githubActivity({
    repos: ["okfn/dashboard", "okfn/openspending"]
    eventsToShow: ['commits', 'issues']
  });
```

@nickstenning any ideas how to do this?
