Avoid forcing dependency injection on class creation by using default parameter values

The current dependency injection model forces to inject the dependencies on all constructors. While useful for the tests, it's cumbersome and inconvenient for the code. Instead, the parameters could be assigned a default value like so:

```python
class CreationService:
def __init__(self, project_loader=ProjectLoader(), template_downloader=TemplateDownloader()):
self.project_loader = project_loader
self.template_downloader = template_downloader
```