[WIP] Kernel Ridge Regression

Adds an implementation of Kernel Ridge Regression (KRR)

Basic implementation of KRR based on @mblondel's code in lightning. In addition to the lighning code, documentation and an example comparing KRR and SVR is added. The example illustrates the models learned by KRR and SVR:

![plot_kernel_ridge_regression_001](https://cloud.githubusercontent.com/assets/1116263/5330658/9793f8f6-7e03-11e4-93a9-7da2f0634037.png)

Furthermore, it compares the time required for fitting the model and predicting with the model for different sizes of the training set:

![plot_kernel_ridge_regression_002](https://cloud.githubusercontent.com/assets/1116263/5330660/a1b7a4cc-7e03-11e4-8628-3d6cccad921a.png)

> either "Reproducing Kernel Hilbert Space (RKHS)" or "reproducing kernel
> Hilbert space (RKHS)"

Scary math concepts like this should be placed at the end of the
documentation of an estimator, as they are not useful to most end users.
Instead, in the beginning of the document, there should be a paragraph
giving the intuition, in layman's terms, of what is different with the
estimator, and why it may be useful.