Please add "user_id" support to CreateOrUpdateSubscriber

[franzo]

See user_id here:
https://www.getdrip.com/docs/rest-api#subscribers

[mpnow]

I've made this change and submitted a PR to merge it into the code base.

[mpnow]

@franzo Drip is in the process of evaluating my PR. In the meantime, if you're desperate, here is my fork which includes providing the user_id when adding or updating a subscriber: Note that I only updated the VS2015 solution, not the VS2013 one (but you shouldn't be using VS2013 any more anyway! 😄).