Re #11; rework memory data store to use hashes
Fix #11; use fewer Redis keys

Now there are 3+N keys, where N is the number of stoplights. They are:

-   stoplight:attempts (hash from strings to integers)
-   stoplight:states (hash from strings to strings)
-   stoplight:threshold (hash from strings to integers)
-   stoplight:NAME:failures (array of JSON strings)
Update changelog
Make failures match between in-memory and Redis data stores
Use the same keys between data stores
Merge branch 'master' into issue-11

Conflicts:
	CHANGELOG.md
	spec/stoplight/data_store/memory_spec.rb
	spec/stoplight/data_store/redis_spec.rb
Fix the specs

Threshold used to be the canonical value. Now it's state.