feat(events): Expose ctx object to the event scripts

Following [this discussion](https://github.com/deployd/deployd/pull/304), this PR will expose the whole ````ctx```` object to the event API.
The ````ctx```` object contains many useful stuff like:

- request
- query / headers
- body
- response
- session
- etc...

Once this is validated, I will  add some documentation about this on this page:
http://docs.deployd.com/docs/collections/reference/event-api.html#s-console.log()-764

Usage example in the ON GET Event:
````
if (ctx && ctx.req && ctx.req.headers && ctx.req.headers.host !== '192.168.178.34:2403') {
    cancel("You are not authorized to do that", 401);
}
````