Add script to set timezone

I added a script to set timezone for container.
This script is able to set any timezone by passing argument.

## Usage

```sh
timezone.sh <timezone>
```

## Example in Dockerfile
```dockerfile
FROM quay.io/wantedly/ruby

ADD <url_for_timezone.sh> timezone.sh
RUN ./timezone.sh Asia/Tokyo
```
You should use `cp` instead of `ln`

Docker has some bug around symlink...
https://github.com/docker/docker/search?q=symlink&type=Issues