Add user_id property to client

[tkdrob]

Pull request summary

Exposes the user_id associated with the token. This is useful as a unique identifier in applications where we only want a single instance configurable by the user.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
  • [x] I have updated the changelog with a quick recap of my changes.
• [ ] This PR fixes an issue.
• [x] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)

[tkdrob]

Missing changelog and documentation updates, otherwise looks good

Sure, no problem. I do have other PRs I want to propose before a version bump. So I will update changelog without a version heading for now.

[IAmTomahawkx]

You can just leave the header as Master, Development etc

[tkdrob]

You can just leave the header as Master, Development etc

I don't see nick referenced in the docs except for the examples so I'm not sure if user_id should be disclosed anywhere else.