add gh-pages generation task

Related to #23 .

I retrieved the content of the current _gh-pages_ branch.
If you want to perform tasks just before the commit on `gh-pages` branch you can do something like:

```dart
void updateGhPages(GrinderContext context) {
  context.log('Updating gh-pages branch of the project');
  new gh.Generator(rootDir: getDir('.').absolute.path)
      ..templateDir = getDir('gh-pages-template').absolute.path
      ..generate(doCustomTask: (workingDir) {
        // copy templates somewhere...
      });
}
```

Basically:
- the content of gh-pages is in the `gh-pages-template` folder.
- to update the `gh-pages` branch run `./grind update-gh-pages` followed by `git push origin gh-pages`.