New feature: get Project Components

Functionality to retrieve Components defined in a Jira project is missing.

Something like:
```
$proj = new ProjectService();
$prjs = $proj->getAllProjects();
// Extract and show Project Components for every Jira Project
foreach ($prjs as $p) {
    var_export($proj->getProjectComponents($p->id));
}
```

I'll try to create a pull request about this.