List field type (fixes #54)

* [x] Basic field definition 
* [x] Implement and tests validators
Because we do not manipulate a real field here.

We just want to check that the ``parameters`` match the ones expected by this field type.

For example : 

```
{
     'name': 'datelist',
     'type': 'list',
     'itemtype': 'date',
     'parameters': {
          'autonow': True
     } 
}
```

Here we want to check that ``autonow`` is a valid parameter, and ``True`` a valid value for it. So we instantiate a fake field : 

```
{
     'name': 'fake',
     'type': 'date',
     'autonow': True
} 
```

And check that it is a valid field definition :)