Add description argument for tasks

Add optional description argument, allowing to easily describe tasks and create help pages, e.g:

```javascript
gulp.task('build', function() {...}, 'Build project');

gulp.task('default', function() {
    for (var task in gulp.tasks) {
        console.log(gulp.tasks[task].name, ' - ', gulp.tasks[task].desc);
    }
});

// Output
build - Build project
```