Add gulp help, a task to list available tasks.

Sometimes when I contribute to a project, I want to run a command similar to `git` (with no args) so that it give me a list of options. I found a little node module to do that.

To try it out, just run `gulp help`:

![gulp-help](https://cloud.githubusercontent.com/assets/254753/4430251/ffab0d6e-4625-11e4-8a46-62fec88d0f37.png)