add python code style checking

[QiJune]

1 use yapf( https://github.com/google/yapf)tool to format python code, and add .style.yapf config file #254
2 contributors can add a git hooks to auto-format changed python code just like this

cat .git/hooks/pre-commit

#!/bin/bash

function python_style_check() {
    #check python code in a git repo
    root=`git rev-parse --show-toplevel`
    exit=0
    for file in `git diff --staged --name-only|grep '\.py$'`; do
        yapf -i --style=$root/.style.yapf $root/$file        
        if [ $? -ne 0 ]; then
            let exit=1
        fi
        git add $root/$file
    done
    exit $exit
}
python_style_check

[coveralls]

Coverage decreased (-0.01%) to 62.432% when pulling b347771 on QiJune:feature/python_code_format into 05204af on baidu:develop.

[reyoung]

@QiJune Please resolve the conflicts first.

[coveralls]

Coverage remained the same at 62.426% when pulling 23a3705 on QiJune:feature/python_code_format into 8d4c453 on baidu:develop.

[reyoung]

Please split this PR into multiple PRs:

• Just checkin code format tools
• Format demo's code
• Format python's code
• ...

Then, please assignment them into different people to review.

This PR make my chrome hang, and cost 2-3G memory to give one line comments.

[reyoung]

comment on this line.

[reyoung]

这个很奇怪呀？直接把\n加到字符串里不就好了

[reyoung]

comment one this line

[reyoung]

Also bad for github, we must shrink PR within 2000-5000 lines, because chrome will cost too many memory.