Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/MailCore/mailcore2
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/CodaFi/mailcore2
Assume main, rather than current, queue for callbacks
Added ObjC-interface
Cleanup and documentation
Merge pull request #431 from CodaFi/assume-main-thread

- Assume main thread.
- Added Obj-C accessors for callbackDispatchQueue.