Fetch tags

This pull request addresses https://github.com/matteosister/GitElephant/issues/55
why don't pass a flag also here, like 

``` php
public function fetch($remote = null, $branch = null, $tags = false)
```

I prefer to have many methods that incapsulate the "knowledge" of calling a command than exposing it to the other classes. It's less error prone for future changes...what do you think?