Add property testing framework for backend

[DKhalil]

We want all our backend services to be property-tested.
There are a couple of candidate libraries (frameworks) which fit the bill:

• excheck
• quixir
• propcheck

[igalic]

propcheck is based on PropEr which makes it a very good candidate, because we could directly reuse learnings from the PropEr Testing Book

[igalic]

<meena> so, anyway, is there a consensus on which one of the three(?) property testing libs are the okayest to start from https://github.com/Brainsware/sealas/issues/1 ?
<gazler> meena: There is also https://github.com/whatyouhide/stream_data which is planned to be merged into Elixir core in 1.6 or 1.7
<gazler> There's a big discussion about it here https://elixirforum.com/t/questions-about-property-testing-stream-data/9445

of those three, it seems to be a fourth :D