update design doc: Client Library of Parameter Server

implement task handling for master server's service

Allow setting both is_static and initial_mean, etc. at the same time

design doc for implementation parameters in CPP.

Add design doc for DeepSpeech2 on PaddlePaddle.

Some time we simply want to have a fixed initialization for a model.

Fix bug in run_python_tests.sh

Fix compile when not enable testing

* Also, test_plot cannot run in MacOS because of matplotlib & ipython's
  limit.
* Add missing dependency in setup.py.
* fix #2264

* WITH_TESTING is a flag that user could configure

"add mq2007 to dataset init"

modify seq2seq demo to show gradient/error clipping.

fix hardcoded cmake for cclient/test

move recordio to Paddle from github.com/wangkuiyi/recordio

enrich paddle built-in learning to rank dataset with the mq2007 dataset

Enable the setting of global gradient clipping threshold