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
Merge pull request #9 from jarsen/master

Stream item updates
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
Implements path method for CKITodoItem
Merge pull request #11 from derrh/master

Implements path method for CKITodoItem
Uses the assignment id for the todo items path
Merge pull request #12 from derrh/master

Uses the assignment id for the todo items path
I have fixed this bug at least 4 times
Merge pull request #13 from derrh/master

I have fixed this bug at least 4 times