* MOD treelet kernel, retrieve canonkeys of all graphs before calculate kernels, wildly speed it up.

* MOD the way to calculate WL subtree kernel, correct its results. - linlin
* ADD *kernel_train_test* and *split_train_test* to wrap training and testing process. - linlin
* MOD readme.md file, add detailed results of each kernel. - linlin

* ADD delta kernel. - linlin
* MOD reconstruction the code of marginalized kernel. - linlin