[WIP] Consider invoking child tasks when invoking a group

This PR is a **suggestion** to *consider* also invoking child tasks when invoking a group.

Example Phakefile:
```php
group('build', function() {
  task('documentation', function() {
    // exec phpdoc/apigen/..
  });
  task('phar', function() {
    // exec box/phar-composer/..
  });
  task('deb', 'phar', function() {
    // bundle above phar as debian package
  });
});

// the default is to invoke all build:* tasks
task('default', 'build');
```

The actual order of *when* to execute the child tasks is of course open to discussion. In my option it makes sense to execute "before", "closure", "children", "after" in this order. But then again maybe I'm overlooking some edge case.

Depends on #23, see 22b9bc4 for the actual changeset.