ENH: improve FA vs PCA example

This improves the plot FactorAnalysis VS PCA example in demonstrating that the selected model's covariance is more appropriate for our low rank scenario than the ones obtained from other covariance estimators.

cf.
![screenshot 2013-10-25 12 48 36](https://f.cloud.github.com/assets/1908618/1407680/abdfa76e-3d73-11e3-86cd-17c4b9789c6d.png)
![screenshot 2013-10-25 12 48 46](https://f.cloud.github.com/assets/1908618/1407681/abe211d4-3d73-11e3-9b73-4bc6ebac0b36.png)