Add script to extract sub-directories' git log

(create_changelog.sh) : Add script to extract sub-directories' git log
according to this comment:
https://github.com/jsk-ros-pkg/jsk_roseus/pull/114#issuecomment-48172724

Usage is here:
```
./create_changelog.sh # for jskeus/eus and jskeus

./create_changelog.sh -s "jskeus/eus jskeus" # specify sub directories
./create_changelog.sh -s "jskeus" # specify sub directories
./create_changelog.sh -s "jskeus/eus" # specify sub directories

./create_changelog.sh -t `rospack find euslisp` # specify directory including CHANGELOG.rst

./create_changelog.sh -s "build/hrpsys-base-source" -t `rospack find hrpsys` # hrpsys example for groovy
./create_changelog.sh -s "build/hrpsys-base-source" -t ~/ros/hydro/build/rtm-ros-robotics/openrtm_common/hrpsys # hrpsys example for hydro, not tested
```