[core] extract BridgeAbstract methods to make them functions

- returnError, returnServerError, returnClientError ,debugMessage are
  moved to lib/error.php

- getContents, getSimpleHTMLDOM, getSimpleHTMLDOMCached are moved to
  lib/contents.php

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

bridges/OpenClassroomsBridge.php

@@ -31,8 +31,8 @@ class OpenClassroomsBridge extends BridgeAbstract{
     }
 
     public function collectData(){
-        $html = $this->getSimpleHTMLDOM($this->getURI())
-          or $this->returnServerError('Could not request OpenClassrooms.');
+        $html = getSimpleHTMLDOM($this->getURI())
+          or returnServerError('Could not request OpenClassrooms.');
 
         foreach($html->find('.courseListItem') as $element) {
                 $item = array();