Add support for Irwin-Hall distribution

[hot9cups]

Go through the Gaussian distribution class, the Binomial class and the Exponential class which are subclasses of General distribution class.

I would like to expand this to include another distribution, the Irwin-Hall distribution.
This one might be slightly more challenging than the other distributions ;)

[whynesspower]

Hi @hot9cups. If this does not get assigned, I would like to complete this task. To prepare, my plan is to go through the code for the distribution classes, their respective tests in test.py, documentation for matplotlib, read up on Irwin Hall distribution, come up with a numbers_irwin_hall.txt for demo data. For the implementation, I'm going to mimic the implementation of the other distribution classes?
Consider assigning me this issue

[hot9cups]

@whynesspower That sounds like a great plan! Just a headsup, I'm moving towards using seaborn instead of matplotlib(see this #63 ), I will merge it soon so the changes will reflect in the codebase as well.
Apart from that, everything sounds great. Assigning this to you :)