[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. 
I would rename (and switch the orders) to:

```python
for InputFeaturesType, TargetType in types:
    # do checks
    ...
```