Fix: Flash export buttons would not work in IE

- Note that this requires the use of an `<a>` tag rather than `<button>` to work in IE. Only in IE is the button tag changed. `<button>` is the correct one for accessibility
- GH #153

Author: AllanJard

js/dataTables.buttons.js

@@ -1148,7 +1148,10 @@ Buttons.defaults = {
 			className: 'dt-button-collection'
 		},
 		button: {
-			tag: 'button',
+			// Flash buttons will not work with `<button>` in IE - it has to be `<a>`
+			tag: 'ActiveXObject' in window ?
+				'a' :
+				'button',
 			className: 'dt-button',
 			active: 'active',
 			disabled: 'disabled'