[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```).
nitpick, you could avoid the zip
```
[(X_ipca, "Incremental PCA of iris dataset ),
 (X_pca,"PCA of iris dataset" )]
```

Maybe you factored out the common part ("of iris dataset").