core: add again() method to timer

gevent/core_.pyx

@@ -579,6 +579,12 @@ cdef public class timer(watcher) [object PyGeventTimerObject, type PyGeventTimer
         def __get__(self):
             return self._watcher.at
 
+    def again(self, object callback, *args):
+        self.callback = callback
+        self.args = args
+        libev.ev_timer_again(self.loop._ptr, &self._watcher)
+        INCREF
+
 
 cdef public class signal(watcher) [object PyGeventSignalObject, type PyGeventSignal_Type]:
 

gevent/libev.pxd

@@ -107,6 +107,7 @@ cdef extern from "libev.h":
     void ev_timer_init(ev_timer*, void* callback, double, double)
     void ev_timer_start(ev_loop*, ev_timer*)
     void ev_timer_stop(ev_loop*, ev_timer*)
+    void ev_timer_again(ev_loop*, ev_timer*)
 
     void ev_signal_init(ev_signal*, void* callback, int)
     void ev_signal_start(ev_loop*, ev_signal*)