People tagging and push updates

This changeset introduces documentation for several new API changes.

Note: I discovered that this page (http://nationbuilder.com/people_tags_api) does not have a home inside this repository. Thus, I rewrote the documentation for a few of those endpoints and added them to the people API page, where they should've originally been.

FYI, this is a public repository, so it's not a good idea to post Trello links and such.

@3dna/web-services