removing repeated code and standardizing the order of parameters

This patch is an attempt to standardize parameter ordering and avoid repeated code. The main changes are

• the introduction of StandardDistributions which define _rvs and _logprob for a couple common functional forms. Where possible, all sampling distributions will either inherit or call these standard methods
• changing the ordering of parameters for objects that do not inherit from the standard distributions so that they nevertheless follow an expected order (e.g., min, max, pow)