Fix #2879 (Check-Icons)

IE11 shows checkmark in wierd position. It seems that IE does size the "checkmark" differently than firefox, which make them appear to be more to the right and bottom. I'm using glyphicons to fix that. The look is constistent in different browser. Though its kind of thicker than the original was.

public/css/tasks.styl

@@ -222,31 +222,29 @@ for $stage in $stages
     content: ''
   label:hover:after,
   label:focus:after
-    content: '¬'
+    content: "\E013"
+    font-family 'Glyphicons Halflings'
     border none
     margin 0
-    font-size: 3.2em
-    line-height: 0.9375em
-    height: 0.9375em
-    width: 1.15em
+    line-height: 1.714285714em
+    height: 1.714285714em
+    width: 1.714285714em
     text-align: center
-    transform: rotate(135deg)
     opacity: 0.5 !important
   label:active:after
     opacity: 0.75 !important
 
   // completed icon
   input[type=checkbox]:checked + label:after
-    content: '¬'
-    font-size: 3.2em
-    line-height: 0.9375em
-    height: 0.9375em
-    width: 1.15em
-    text-align: center
-    transform: rotate(135deg)
-    opacity: 0.75
+    content: "\E013"
+    font-family 'Glyphicons Halflings'
     border none
     margin 0
+    line-height: 1.714285714em
+    height: 1.714285714em
+    width: 1.714285714em
+    text-align: center
+    opacity: 0.75
 
 
 // secondary task commands
@@ -477,4 +475,4 @@ form
 .repeat-days
   a
     border-radius: 0.382em
-          
+