Use simplejson first, and centralize import

simplejson is supposed to be faster:
http://stackoverflow.com/questions/712791/json-and-simplejson-module-differences-in-python

modules/radiofrance/browser.py

@@ -18,6 +18,7 @@
 # along with weboob. If not, see <http://www.gnu.org/licenses/>.
 
 from weboob.tools.browser import BaseBrowser, BasePage
+from weboob.tools.json import json
 from weboob.capabilities.video import BaseVideo
 from weboob.tools.browser.decorators import id2url
 
@@ -25,11 +26,6 @@ from StringIO import StringIO
 from time import time
 import re
 
-try:
-    import json
-except ImportError:
-    import simplejson as json
-
 try:
     from urlparse import parse_qs
 except ImportError: