Add dependency to gflags and related tests

Add pre-commit hook for contribute documentation.

* Also add a soft link for contribute.md to make github recognize
  this file.

add exclude_patterns for conf.py.in

add Chinese document for demo imagenet_model

Fit pre-commit for clang-format 4.x

Integrate doc en/cn into single doc

Fix enable_virtualenv

The resource "unsigned char[]" array generated by create_source needs to have 0 ending as a valid C string to be used in PythonUtil.cpp initPython()

Change-Id: I5e214606ed9102f37813ea3f07565dc10a9c015e

Fix warning

Fix Travis-CI build error.

* error because merge of #711. The old issue don't check pre-commit
  hooks.

Remove m4 when generate protobuf

adding input type check for python data provider

* Also fix a protential bug in WarpCTCLayer.cpp

fix some dead links in doc/

Also fix compile issues

add submodule instructions in build_and_install doc