Enable the `RestTool` class to be customized

Add a proxy around the `RestTool` class to let the user swap it.

So with this Pull Request we can do something like

```python
from mangopaysdk.tools.resttool import BaseRestTool

class MyOwnRestTool(BaseRestTool):
    def Request(self, *args, **kwargs):
        logger.info("pre-request ...")
        response = super(MyOwnRestTool, self).Request(*args, **kwargs
        logger.info("post-request ...")
        return response

Configuration.ClientID = '...'
Configuration.ClientPassword = '...'
Configuration.RestToolClass = MyOwnRestTool
```

An example of use case is if you want to log every request made and every response ever made by the mangopaysdk, now you can subclass the `BaseRestTool` like shown above and implement your logic.

I'll add some commits in this PR to improve the possible customizations.

Cheers