fix(uiGridColumnMenu): Position relatively

Positioning absolutely worked in all browsers but IE9. It was positioning
the column menu at the top of the page...

Changing to relative seems to have fixed this in IE9 and kept all other
browsers the same.

Fixes #2319

src/less/header.less

@@ -1,6 +1,6 @@
 
 .ui-grid-top-panel-background {
-  .gradient(@headerBackgroundColor, @headerGradientStart, @headerGradientStop);  
+  .gradient(@headerBackgroundColor, @headerGradientStart, @headerGradientStop);
 }
 
 @topPanelRadius: @gridBorderRadius - @gridBorderWidth;
@@ -17,7 +17,7 @@
     overflow: hidden;  // Disable so menus show up
     font-weight: bold;
 
-    // .gradient(@headerBackgroundColor, @headerGradientStart, @headerGradientStop);  
+    // .gradient(@headerBackgroundColor, @headerGradientStart, @headerGradientStop);
     .ui-grid-top-panel-background;
 
     .border-radius(@topPanelRadius, 0, 0, @topPanelRadius);
@@ -103,7 +103,7 @@
 }
 
 .ui-grid-column-menu {
-  position: absolute;
+  position: relative;
 }
 
 /* Slide up/down animations */
@@ -112,12 +112,12 @@
     .transition(all, 0.05s, linear);
     display: block !important;
   }
-  
+
   &.ng-hide-add.ng-hide-add-active,
   &.ng-hide-remove {
     .transform(translateY(-100%));
   }
-   
+
   &.ng-hide-add,
   &.ng-hide-remove.ng-hide-remove-active {
     .transform(translateY(0));
@@ -130,12 +130,12 @@
     .transition(all, 0.05s, linear);
     display: block !important;
   }
-  
+
   &.ng-hide-add.ng-hide-add-active,
   &.ng-hide-remove {
     .transform(translateY(-100%));
   }
-   
+
   &.ng-hide-add,
   &.ng-hide-remove.ng-hide-remove-active {
     .transform(translateY(0));