[MRG] 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```).
This looks good, do you get any warning when building? Are you sure you have the right `sklearn` module in your `site-packages` when you run `make html`? You can use `pip uninstall scikit-learn && pip install -e /path/to/scikit-learn` to make sure (or equivalently `python setup.py develop` from within  `/path/to/scikit-learn`).