Add `options` param to event requests

Event requests now accept a full option in order to return the whole response.

This is useful to read the response `ETag` header used to make subsequent requests to event requests, see http://developer.github.com/v3/activity/events/

Example:

```ruby
events = Github::Activity::Events.new
response = events.repository 'user-name', 'repo-name', {}, { :full => true }
response.headers['ETag']
```