Fix IE leaks caused by ._fireEvent circular reference

changed empty, now it uses destroy to clean every children tree
this change need a check on destroy, because childNodes returns
textNodes
on dispose i removed the _fireEvent reference breaking the circular
reference and allowing the gc to clean up the memory (tested with
sIEve-0.0.8)
set('html',...) don't leak but set('text',...) does, so added a
Element.Properties.text setter and done empty() before setting the text

I setup a test page that creates 1000 divs, added these divs to the
body and then use empty(), set('html', ...) and set('text', ...) to get
rid of them, causing a 1000 element leak,
with these changes I managed to drop the leaks from 1000 to 0.

Source/Element/Element.js

@@ -800,18 +800,20 @@ var formProps = {input: 'checked', option: 'selected', textarea: 'value'};
 Element.implement({
 
 	destroy: function(){
+        if(! this.getElementsByTagName) return null; //textNode
 		var children = clean(this).getElementsByTagName('*');
 		Array.each(children, clean);
 		Element.dispose(this);
 		return null;
 	},
 
 	empty: function(){
-		Array.from(this.childNodes).each(Element.dispose);
+		Array.from(this.childNodes).each(Element.destroy);
 		return this;
 	},
 
 	dispose: function(){
+        this._fireEvent = null;
 		return (this.parentNode) ? this.parentNode.removeChild(this) : this;
 	},
 
@@ -949,6 +951,13 @@ Element.Properties.html = {
 
 };
 
+// fix for IE leak on Element.set('text','')
+Element.Properties.text = {
+    set: function(text){
+        Element.prototype.empty.call(this).setProperty('text',text);
+    }
+}
+
 var supportsHTML5Elements, supportsTableInnerHTML, supportsTRInnerHTML;
 
 /*<ltIE9>*/