Add dependency for memecached
Accept argument for global caching
Pass around memcached client

Surely there must be a better way to do this.
As things stand, every new argument need to be
passed all the way down to Request.
Cache response if possible
Flush cache whenever new server is started
Store cached time, and Response object in memcache

Cached time will be used to determine Age.
Response object is stored instead of raw response because
so that headers can be changed after retrieval from cache.
Add Age header to cached response
Explicitly add timeout for cache
Add installation instructions
Merge branch 'feature/simple-caching' into develop