[WIP] Validation curves

... the second part of #2584.

Here is an example:

```python
from sklearn.datasets import load_digits
from sklearn.svm import SVC
from sklearn.learning_curve import validation_curve
 
digits = load_digits()
X, y = digits.data, digits.target
 
param_range = np.logspace(-5, -1, 5)
train_scores, test_scores = validation_curve(
    SVC(), X, y, gamma=param_range, cv=10, scoring="accuracy")
 
semilogx(param_range, train_scores)
semilogx(param_range, test_scores)
```