add Json.keys()
fix Json.keys() test
add libevent to 'stable branch'
patched from ide-san
delete libevent_event_glue.c
delete comment out line
import 'rename KKMethodName_ => KMethodName_' from stable branch
rename Libevent => Event_base
rename Libevent_event => Event
rename 'Event*' => 'CEvent*' in 'libevent_glue.c'
[Temporary] before delete 'Event' class in glue.k
implement konoha callback
Fix a few mistake.

in this commit, test.k says...
 - (error) (libevent_glue.k:40) Func[void] is requested, but Func[void,Object] is given
[Temporary commit] event_add() success from test.k
[FIX] success to event_add().
however, still occurs segmentation fault in other reason.

-----
 % ./minikonoha.sh build.debug libevent/sample/test.k
  - (test.k:22) --- 1 ----
  cEvent_base->event_base:(nil), evd:4, event:0x12, callback_1st:0x7fc3436e6257, ev:0x7fc3445816c0
  ev->event = 0x98f100 in CEvent_new
  kcev->event = 0x98f100 in System_event_add
  [warn] event_add: event has no event_base set.
  System_event_add 2
   - (test.k:24) --- 2 ----
   Segmentation fault (core dumped)
-----
[FIX] success to konoha-callback execute.
[TODO] However, Map.set() occurs SEGV
rename libevent_glue.k => libevent_kick.k
to work constructor
success parameter passing to konoha callback.
[FIX] CEvent_base.event_dispatch() return type.
delete 'arg' member in kCEvent.