[WIP] Incremental PCA

An implementation of Incremental PCA. See [Incremental Learning for Visual Tracking, J. Lim, D. Ross, R. Lin, M. Yang, 2004](http://www.cs.toronto.edu/~dross/ivt/LimRossLinYang_nips04.pdf) for more details.

The key idea behind this PR is to have an out-of-core PCA that gives roughly equivalent results while using constant memory (on the order of the ```batch_size``` rather than ```n_samples```).
Avoid using `\`. You could break string as

```
raise ValueError("n_components=%r invalid for n_features=%d, need more rows "
                 " than columns for IncrementalPCA processing "
                 % (n_components, n_features))
```