add Nginx::Time

We can do same things by `mruby-time` also.
But `Nginx::Time` uses nginx-self-timer.
So `Nginx::Time` can avoid the overhead of time-syscall as compared to `mruby-time`.

Look the following page about how to use `Nginx::Time`.

http://cubicdaiya.github.io/mruby_nginx_module/classes.html#toc_4