Make docstring examples in stats.distributions docstrings runnable

closes gh-2106.

I've tweaked the default example a bit: removed the semilog plot of cdf-ppf roundtrip (cf gh-1946), and replaced it with an example from the test suite. Tried adding an example of using the named arguments, but did not manage to convince the `doccer` to respect indentation. The resulting example is definitely not perfect, but at least it is valid python. 

For the moment I'm leaving a slew of commits as is, would be happy to squash them away if there are strong opinions.