Replace anchor point/range with a single concept: markers

A "marker" is basically like a persistent selection/cursor composite,
having a head and a tail. The "head" is like the cursor in a selection,
and the "tail" is like the part of the selection that doesn't move. My
goal is for markers to be the only construct used to track regions
in the buffer. I want to replace anchors with them.

src/app/edit-session.coffee

@@ -9,7 +9,6 @@ EventEmitter = require 'event-emitter'
 Subscriber = require 'subscriber'
 Range = require 'range'
 AnchorRange = require 'anchor-range'
-AnchorPoint = require 'anchor-point'
 _ = require 'underscore'
 fs = require 'fs'