Examples [WIP]

Ok, next pull request coming up!

I've build a buggy version of a quicksort implementation with a quite common bug (neither the `smaller_than` nor the `larger_than` part include the "equal" part). QuickCheck will find this bug.

I also added a more descriptive name to the invariant on your `reverse` example. I hope you like it :wink: 

**Work in Progress:** I don't know how elegant my implementation is. I tried to keep it to my Haskell implementation, which is only 5 lines. This implementation gets kinda messy with conversions between vectors and ranges. I will ask for feedback in the Rust IRC :+1: