[MRG] Validation curves

... the second part of #2584.

Here is an example:

```python
import numpy as np
from sklearn.datasets import load_digits
from sklearn.svm import SVC
from sklearn.learning_curve import validation_curve
import matplotlib.pyplot as plt

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, param_name="gamma", param_range=param_range,
    cv=10, scoring="accuracy", n_jobs=4)

plt.semilogx(param_range, train_scores)
plt.semilogx(param_range, test_scores)
plt.show()
```