TST Adding two unittests for testing GP kernels
TST Adding two unittests for testing GP kernels

test_kernel_versus_pairwise() checks that GP kernels can also be used as pairwise kernels.
test_set_get_params() checks that set_params()/get_params() is consistent with kernel.theta.
fabianp/scikit-learn,fabianp/scikit-learn,fabianp/scikit-learn,fabianp/scikit-learn