Feature/travis pre commit checks

Fix bugs in Docker document

Fix Travis-CI when using rebase.

* The $TRAVIS_COMMIT_RANGE could be not existed, because the git history
  can be changed and rebased. Here we alway run Travis-CI when `git diff`
  cannot return the modified files.
* Also do not push docs update if is not PaddlePaddle/Paddle.

Fix dead links in quick start after reorganzing documentaion directory

Bazel protobuf and gtest

Do not use https://github.com/pubref/rules_protobuf, but use https://github.com/google/protobuf/ as external dependency for protobuf

Remove sudo from Dockerfile.*

test code for issue #729

remove breathe and doc/howto/source directory

Support predicting the samples from sys.stdin

Fix dead link after rearrange english documentation.

Correctly handling multiple calls to parse_config()

Install libgtest-dev in Dockerfiles

add semantic_role_labeling_cn.md

* Fix #803