Add log-cdf methods for all distributions

[fonnesbeck]

Some have pointed to the need for cumulative distribution functions, for example, to rescale censored PDFs. To this end, we should add logcdf methods to each of our distributions. Since we will probably have to add these over time, I have created a cdf-methods branch to submit pull requests to. Once we get them working for all of the distributions, we can merge them to master.