Changes from Baobab Development

There are a few new features and bug fixes in this branch, including:
- the Inbox App ID is exposed in addition to the BaseURL
- updated attachments logic: attachment JSON is now returned in the request for messages, so attachmentIDs -> attachmentData (similar to tagData on threads)
- tag interactions are now supported on threads
- you can fetch the drafts of a thread in addition of the messages of a thread
- preliminary support for downloading attachments (via https://github.com/eligrey/FileSaver.js/, not included)
1. new function is getting in between the docs and implementation for `INMessage#attachment()`, which we probably don't want to do.

2. `attachmentsPromise()` might be a bit of a weird name, doesn't really reflect what it's doing. Maybe `getAttachments()` or something?