core: removed pyfeed and xe dependencies:

pyfeed and xe where still used only for date format handling, and there is already dateutil which is a dependency of Wokkel.
    Furthermore pyfeed and xe are still not on pypi, causing troubles for installation with pip

src/tools/utils.py

@@ -43,6 +43,7 @@ def xmpp_date(timestamp=None, with_time=True):
     """Return date according to XEP-0082 specification
 
     to avoid reveling the timezone, we always return UTC dates
+    the string returned by this method is valid with RFC 3339
     @param timestamp(None, float): posix timestamp. If None current time will be used
     @param with_time(bool): if True include the time
     @return(unicode): XEP-0082 formatted date and time