Initial release

Another repo, another day. This is the initial release of `request-redis-cache`, an store for caching external requests in Redis.

More information can be found in the README: https://github.com/uber/request-redis-cache/tree/dev/initial.release.sqwished

In this PR:

- Added `request-redis-cache` library
- Added tests for functionality and edge cases (e.g. Redis is acting up)
- Added documentation

/cc @mlmorg @Raynos