Modify the arguments description of ConvFunctionBase. And add the definition of backward input and backward filter function.

Add user can define PROTOBUF_ROOT.

Support variable-dimension input feature for 2D convolution operation.

Expose the "reverse" argument for recurrent_group in V2 API.

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