Ben Morgan authored Aug 15, 2018 and Brad King committed Aug 20, 2018

The `avatar_url` key for users and projects may be null if GitLab uses
its internal or default settings.  Change struct members for
`avatar_url` to `Option<>` to represent this possibility.

Since gitlab-ce commit 85cdde8d33 (Migration - Remove position field
from issues and merge requests, 2017-06-09) the webhook object
attributes for issue and merge request events no longer have a
`position` field.

Fetch api v4 sample data from a GitLab 9.2 instance.  Using that and
GitLab sources `doc/api/v3_to_v4.md` and `lib/api/entities.rb`, update
our types accordingly.  Most changes are to rename "builds" to "jobs" or
"pipelines".

Also add strong types to internal ids in preparation for switching to
them when constructing API endpoints.

The chrono 0.3.1 release was yanked from crates.io due to breaking
semantic versioning (chrono issue 156).  Now chrono 0.4 has been
released to support serde 1.0.

This allows the `strict` feature to actually be used. Most of these
fields are deprecated and are therefore not made public.

Useful for debugging, but should not be used in production.

Fixes #4.

Got bored typing up docs part of the way through. Some of the more
obscure entities are left untouched.

Objects meant to be passed to POST or PUT endpoints are not represented
here (though they should be added).

Fixes #1.