Merge in global properties.

At remind101, we run most of our apps on heroku, and we like to propagate the [request_id throughout our system](https://github.com/remind101/request_id). This change allows you to set some default properties, so that you can do things like set headers at a global level:

```ruby
Hutch.global_properties = proc {
  { app_id: 'api', headers: { request_id: RequestId.request_id } }
}
```