Change input type of search field to "search"

This generates a simple clear element when typing. Cloeses https://github.com/WhisperSystems/Signal-Desktop/issues/420

background.html

@@ -13,7 +13,7 @@
       <span class='socket-status'></span>
       <div class='tool-bar clearfix'>
         <!-- <button class='show-new-conversation'></button> -->
-        <input type='text' class='search' placeholder='Search for people or groups'>
+        <input type='search' class='search' placeholder='Search for people or groups'>
       </div>
     </div>
     <div class='gutter'>