"Burn after reading" as a checkbox

"Burn after reading" option has been moved out of Expiration combo to a
separate checkbox.
Reason is: You can prevent a read-once paste to be available ad vitam
eternam on the net.

(cherry picked from commit 190b278402c086ebc4d1a78aae27d1e2666e3e7a)

Conflicts:
	css/zerobin.css
	index.php
	js/zerobin.js
	tpl/page.html

css/zerobin.css

@@ -178,7 +178,7 @@ button img {
 	top: 2px;
 }
 
-#expiration, #language, #opendisc { 
+#expiration, #language, #burnafterreadingoption, #opendisc { 
 	background-color: #414d5a; 
 	padding: 6px 8px;
 	margin: 0 5px 0 0;