[MRG] better Pandas support.

Don't convert dataframes in grid search and cross-validation.

If we can agree on this, I'll
* [x] add tests
* [x] rename allow_lists to force_array (and invert the meaning). It's an internal parameter anyhow. 
Assuming the ordering switch, I rather would put:

```python
    types.append((Dataframe, Series))
    types.append((Dataframe, Dataframe))
```

and simplify the checking code.