Add support for SVDD (one-class SVM) #2807

Support Vector Data Description (SVDD) could be a nice enhancement to OneClassSVM implementation.

A technical implementation is described in this paper:
http://www.csie.ntu.edu.tw/~cjlin/papers/svdd.pdf

Source code compatible with libsvm is available here:
http://www.csie.ntu.edu.tw/~cjlin/libsvmtools/#libsvm_for_svdd_and_finding_the_smallest_sphere_containing_all_data

I have modified the sklearn/svm/src/libsvm/svm.h and svm.cpp files to add additional functionality of SVM.

-s 5 SVDD
-s 6 R^2 L1SVM
-s 7 R^2 L2SVM