Simplify interface

[charleskawczynski]

One of the nice parts of Julia is that we can extend existing methods, for example,

get_mean(d::Parameterized) = mean(d.distribution)

could simply extend the StatsBase mean method

import StatsBase
...
StatsBase.mean(d::Parameterized) = StatsBase.mean(d.distribution)

This can be applied to several methods (e.g., get_mean, len, size etc.)

[ilopezgp]

Let's go through these before release 0.2, since some of these will be breaking changes.