Enable passing Laplace smoothing parameter from higher level APIs

[sriramvasudevan]

The skew computation and KL divergence metrics accept an alpha parameter to add Laplace smoothing. However, this parameter is not exposed via the higher level APIs. We should be able to supply a custom value of smoothing to apply, since the default of add one smoothing is not always appropriate for a given distribution (eg: add one smoothing for a probability distribution skews it more towards a uniform distribution, while the same smoothing is okay when we're dealing with a distribution of counts).