Add support for concept of creator of an activity

Hi,

I don't know if this useful for other people or very specif to our domain, but I thought in making the pull request and have your feedback. So our use case was the following. We have organizations that have users. In our context, the owners of the activities are the organizations but we want it to track whom in that organization created the activity. So we can have things like:

User A from Organization B started following Organization C. 

In order to support this feature, I added this concept of creator of an activity that allows me to do things like:
```ruby

  class Organization
    ...
    include PublicActivity::Common
  end

```
And then in the controller:
```ruby
Organization.create_activity :follower,  owner: current_organization,  creator: current_user
```

Cheers :)