[WIP] FIX normalize to unit variance in [sparse_]center_data

It appears `center_data` had been normalizing to variance=`n_samples` since https://github.com/scikit-learn/scikit-learn/commit/295dc3c#diff-7b11510996b18ecc1e4bfd4f4ec850b4R67

At master:
```python
>>> X1 = np.random.normal(size=(1000, 1))
>>> X2 = np.random.normal(size=(10000, 1))
>>> center_data(X1, np.zeros(X1.shape[0]), True, True)[0].std()
0.031622776601683784
>>> center_data(X2, np.zeros(X1.shape[0]), True, True)[0].std()
0.0099999999999999794
```

This also deletes `sparse_std`, cleans up and tests the centering code.