Maybe add truncated normal

[ev-br]

As requested in https://github.com/scipy/scipy/issues/5888 (which is how I learned about paramnormal, btw).

Again, I'm only a messenger here, the actual request was made by @mountaindust.

Finally, please let me know if my dumping of assorted bits and pieces from scipy.stats discussions is too much --- I'll stop at once :-)

[phobson]

@ev-br I love that even someone has interest in paramnormal. I can't promise that I'll be able to get to everything very quickly -- but I'm sure you understand how that goes.