Added global event API

This adds the global events feed API.

However, I do not recommend merging this until other problems have been solved (mainly authentication).

If someone wants to use the API today, use this patch. But existing code bases may need complex changes, so before merging into master it should be considered in more detail.

Several problems with merging this:

- A new version of Aeson with this patch is needed: https://github.com/bos/aeson/pull/169
- Existing code using the `Event` type won't work. It has been renamed `RepositoryEvent`. Maybe it should be called `IssueEvent`. However, I doubt anyone utilises the issue's event's API since there are no authenticated versions of the functions.
- The data type for Event is not that useful, it just carries a JSON payload.