Add support for worker type to CountercacheRedis

Redis is a shared cache, and using it with multiple HerokuScalers
scaling different worker types causes wrong information to be cached.
This adds support for specifying the worker type, so individual caches
can be maintained.

By default HerokuScaler uses a new MemoryCounterCache, so no caches are
shared.
Merge pull request #34 from bkudria/countercache_worker_types

Add support for worker type to CountercacheRedis