Updated to support conversation messages

Includes changes to support assignment submissions interleaved
with submissions
adds attachment ids to createMessage:
Allow a conversation to merge in sent message
Sets the correct context on assignments for todo items
Adding read_state to CKIActivityStreamItem
Adds a couple of conversation related endpoints.
Implements path method for CKITodoItem
Uses the assignment id for the todo items path
I have fixed this bug at least 4 times
Add URL property to CKIActivityStreamItem
Add necessary url transformer to CKIActivityStreamItem
Update README.md
Add class-specific IDs to stream items.
Fix ID properties in submission stream items.
Updates conversation endpoints to include attachments

Also implemented a method needed to archive messages.
Better document activity stream messages.
Fixes mantle parsing issue with discussion topics

Also added the path for a discussion topic and included in umbrella header.
Added params to the course favorites api so it will get the course grade info as well.
Merge remote-tracking branch 'instructure/master'

Conflicts:
	CanvasKit/Networking/CKIClient+CKIDiscussionTopic.h
	CanvasKit/Networking/CKIClient+CKIDiscussionTopic.m
Added Assignment Groups endpoint