add a script to check code style

scripts/check_code_style.sh

+#!/bin/bash
+function abort(){
+    echo "Your change doesn't follow PaddleHub's code style." 1>&2
+    echo "Please use pre-commit to check what is wrong." 1>&2
+    exit 1
+}
+
+trap 'abort' 0
+set -e
+
+cd $TRAVIS_BUILD_DIR
+export PATH=/usr/bin:$PATH
+pre-commit install
+
+if ! pre-commit run -a ; then
+    git diff
+    exit 1
+fi
+
+trap : 0