fix up some drag event issues

- Change `drag.canDrag` to `drag.enabled`
- Track when dragging ends to reset the delta measure point
- Remove dragstart event which won't fire in the host

h/js/host.coffee

@@ -12,10 +12,9 @@ class Annotator.Host extends Annotator
   # Drag state variables
   drag:
     delta: 0
+    enabled: false
     last: null
     tick: false
-    #Do we enable dragging
-    canDrag: false
 
   constructor: (element, options) ->
     super
@@ -155,7 +154,8 @@ class Annotator.Host extends Annotator
         )
 
         .bind('setDrag', (ctx, drag) =>
-          @canDrag = drag
+          @drag.enabled = drag
+          @drag.last = null
         )
 
   scanDocument: (reason = "something happened") =>
@@ -207,21 +207,12 @@ class Annotator.Host extends Annotator
           @panel?.notify method: 'publish', params: 'hostUpdated'
     document.addEventListener 'touchmove', update
     document.addEventListener 'touchstart', =>
-      unless @canDrag then return
       touch = true
       @frame?.css
         display: 'none'
       do update
     document.addEventListener 'dragover', (event) =>
-      unless @canDrag then return
-      if @drag.last?
-        @drag.delta += event.screenX - @drag.last
-      @drag.last = event.screenX
-      unless @drag.tick
-        @drag.tick = true
-        window.requestAnimationFrame this._dragRefresh
-    document.addEventListener 'dragleave', (event) =>
-      unless @canDrag then return
+      unless @drag.enabled then return
       if @drag.last?
         @drag.delta += event.screenX - @drag.last
       @drag.last = event.screenX