Add usage with -h|--help flag

As advised by the [GNU coding standard](http://www.gnu.org/prep/standards/standards.html#g_t_002d_002dhelp), add some usage with `-h|--help` flag.

It got me crazy to have to go the website documentation to just remember the option to specify a custom `gulpfile.js`

![screen shot 2014-06-13 at 14 26 50](https://cloud.githubusercontent.com/assets/345495/3269862/44101af4-f2f6-11e3-8895-d813228e46dc.png)

`&& tasks.length === 0`
It would be better if this message appears with some new method — gulp.help() for example.
Some plugins make it possible to show a help and I want to show it all together in my tasks via `--help` flag.