added possibility to override function poweredby

bibtexbrowser.php

@@ -2455,6 +2455,7 @@ class IndependentYearMenu  {
   }
 }
 
+if (!function_exists('poweredby')) {
   /** Returns the powered by part. @nodoc */
   function poweredby() {
     $poweredby = "\n".'<div style="text-align:right;font-size: xx-small;opacity: 0.6;" class="poweredby">';
@@ -2463,7 +2464,7 @@ function poweredby() {
     $poweredby .= '</div>'."\n";
     return $poweredby;
   }
-
+}
 
 /** ^^adds a touch of AJAX in bibtexbrowser to display bibtex entries inline.
    It uses the HIJAX design pattern: the Javascript code fetches the normal bibtex HTML page