Updated to support conversation messages

Includes changes to support assignment submissions interleaved
with submissions
Merge remote-tracking branch 'mwright/master'
adds attachment ids to createMessage:
Allow a conversation to merge in sent message
Sets the correct context on assignments for todo items
Adds a couple of conversation related endpoints.
Merge remote-tracking branch 'instructure/CanvasKit/ReactiveCocoa'
Merge pull request #10 from derrh/master

Adds endpoints and model related to conversations