Task description support via forked orchestrator

Created by: evindor

I've made very simple and straightforward support for tasks listing and description. With this pull request to orchestrator it's possible to pass a string as a last argument to gulp.task:

gulp.task('build', function() {...}, 'Build my project');

This PR adds a help task by default, so running gulp help will produce the following output if help task is not overridden in gulpfile:

$ gulp help
[gulp] Using file ...
[gulp] Working directory changed to ...
[gulp] Running 'help'...

Available tasks:
 - help
 - build - Build my project

[gulp] Finished 'help' in 254 μs

This PR is only to start a discussion, i'll update docs and tests if we agree on this approach.