[scikit-learn] Adding samplers for intersection/Jensen-Shannon kernels

[avn at mccme.ru]

Hello,

In the course of my work, I've made samplers for 
intersection/Jensen-Shannon kernels, just by small modifications to 
sklearn.kernel_approximation.AdditiveChi2Sampler code. Intersection 
kernel proved to be the best one for my task (clustering Docstrum 
feature vectors), so perhaps it'd be good to add those samplers 
alongside AdditiveChi2Sampler? Should I proceed with creating a pull 
request? Or, perhaps, those kernels were not already included for some 
good reason?

With best regards,
-- Valery