add Nginx.redirect to serve 2 kinds redirect

add Nginx.redirect to serve 2 kinds redirect:
1st,serve http 3xx redirection.
2nd,serve nginx internal redirection.

test 1:http 3xx redirection

```nginx
# redirect
# http://localhost/redirect?url=http://163.com
location /redirect {
    mruby_rewrite_handler_code '
        r = Nginx::Request.new()
        if r.var.arg_url
            Nginx.redirect r.var.arg_url
        else
            Nginx.redirect "http://taobao.com", 301
        end
    ';
}
```

test 2:nginx internal redirection, such as rewrite syntax in nginx.

```nginx
# internal redirect
# http://localhost/redirect/internal?version=xxx
location /redirect/internal {
    mruby_rewrite_handler_code '
        Nginx.redirect "/vars"
    ';
}
```

test3:dynamic generate location and let nginx serve it, vary useful for dynamic location + static files.

```nginx
# dynamic the internal path for your business
# http://localhost/redirect/internal/dynamic/path?path=hello
# actually this request will served by location /static/
location /redirect/internal/dynamic/path {
    mruby_rewrite_handler_code '
        r = Nginx::Request.new()
        dynamic_path = "/static/#{r.var.arg_path}"
        Nginx.redirect dynamic_path
    ';
}

location /static/ {
    alias /path/to/static/;
}
```