text: Title: Docs are not clear about installation requirements
  Body: The docs say:
  
  > Eve is powered by Flask, Redis, Cerberus, Events
  
  but it does not indicate if all of those are required.
  
  Specifically, I have failed to find anywhere in the docs if Redis is an optional dependency.
  
  Looking into the requirements.txt and reading usage samples of Redis, `app = Eve()`  vs `app = Eve(redis=r)` also suggests Redis is optional.
  
  However, that is too many hoops for those new to Eve to conclude about Redis requirement - many may give up having no option to host Redis. For example, PythonAnywhere users: [I couldn't tell from their docs whether redis is a hard requirement or something that you can use for some features](https://www.pythonanywhere.com/forums/topic/3730/#id_post_18968)).
  