Merge pull request #17 from derrh/master

Updates conversation endpoints to include attachments
Better document activity stream messages.
Merge pull request #18 from jarsen/master

Better document activity stream messages.
Merge pull request #19 from instructure/ReactiveCocoa

Reactive Cocoa
Fixes mantle parsing issue with discussion topics

Also added the path for a discussion topic and included in umbrella header.
Merge remote-tracking branch 'instructure/CanvasKit/master'