ENH: Add Redis IO functionality
Adds two primary IO methods: to_redis() and read_redis() 

TL;DR: Serializes DataFrames in and out of Redis. I've used a version of this code in production previously to share a cached DF across multiple worker processes. 

- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry
