[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/LeBonCoinBridge.php

@@ -143,11 +143,11 @@ class LeBonCoinBridge extends BridgeAbstract{
             $category='annonces';
         }
 
-        $html = $this->getSimpleHTMLDOM(
+        $html = getSimpleHTMLDOM(
             self::URI.$category.'/offres/' . $this->getInput('r') . '/?'
             .'f=a&th=1&'
             .'q=' . urlencode($this->getInput('k'))
-        ) or $this->returnServerError('Could not request LeBonCoin.');
+        ) or returnServerError('Could not request LeBonCoin.');
 
 		$list = $html->find('.tabsContent', 0);
 		if($list === NULL) {