Make C lib in `paddle.v2.master.client` lazy load

Currently, WITH_GOLANG is a compiling flag, and not all Paddle binary
enable `WITH_GOLANG=ON`. So we cannot ensure that every paddle has
`master.so`. Make load that C libary lazily could let the user run
Paddle without `WITH_GOLANG=ON`.

python/paddle/v2/master/client.py

 import ctypes
 import os
 
-path = os.path.join(os.path.dirname(__file__), "libpaddle_master.so")
-lib = ctypes.cdll.LoadLibrary(path)
+__lib__ = None
+
+
+def get_c_lib():
+    if __lib__ is None:
+        global __lib__
+        path = os.path.join(os.path.dirname(__file__), "libpaddle_master.so")
+        __lib__ = ctypes.cdll.LoadLibrary(path)
+    return __lib__
 
 
 class client(object):
@@ -11,8 +18,8 @@ class client(object):
     """
 
     def __init__(self, etcd_endpoints, timeout_sec, buf_size=0):
-        self.c = lib.paddle_new_etcd_master_client(etcd_endpoints, timeout_sec,
-                                                   buf_size)
+        self.c = get_c_lib().paddle_new_etcd_master_client(
+            etcd_endpoints, timeout_sec, buf_size)
 
     def request_save_model(self, trainer_id, block_ms):
         """request to save model
@@ -32,10 +39,11 @@ class client(object):
             saving the model, -1 if error happened.
 
         """
-        return lib.paddle_request_save_model(self.c, trainer_id, block_ms)
+        return get_c_lib().paddle_request_save_model(self.c, trainer_id,
+                                                     block_ms)
 
     def release(self):
-        lib.paddle_release_master_client(self.c)
+        get_c_lib().paddle_release_master_client(self.c)
         self.c = None
 
     def set_dataset(self, paths):
@@ -45,7 +53,7 @@ class client(object):
         for idx, path in enumerate(paths):
             c_ptr = ctypes.c_char_p(path)
             holder[idx] = c_ptr
-        lib.paddle_set_dataset(self.c, holder, len(paths))
+        get_c_lib().paddle_set_dataset(self.c, holder, len(paths))
 
     def next_record(self):
         """gets next record for training
@@ -56,7 +64,7 @@ class client(object):
         """
         p = ctypes.c_char_p()
         ret = ctypes.pointer(p)
-        size = lib.paddle_next_record(self.c, ret)
+        size = get_c_lib().paddle_next_record(self.c, ret)
         if size < 0:
             # Error
             return None, size
@@ -67,5 +75,5 @@ class client(object):
 
         record = ret.contents.value[:size]
         # Memory created from C should be freed.
-        lib.mem_free(ret.contents)
+        get_c_lib().mem_free(ret.contents)
         return record, 0