Add PrettyAnn type class - #256

Thank you for getting this moving! Looks good to me.

FYI, I hope to get a few new co-maintainers involved in this project (see #272). This PR will be merged if the new maintainers approve it.

(And of course: If you're interested in becoming a co-maintainer, please let me know! :-) )

Given that this has been approved by so many people prior to me already, I think this is good to go. When we release we'll mark all the FIXME's with 1.7.1.

I'll let @Ei30metry be the final say, since this is the first PR we're overseeing as co-maintainers.

Thanks @a-02, here are my thoughts:

1. The Pretty class has many examples, we should probably add some haddock comments if possible. Even mentioning some of the example annotations would go a long way.
2. The other one is Pretty instances for 4-tuple, 5-tuple, and 6-tuple #274

Everything else looks great. Thanks for the contribution @mmhat!