add doctrine/cache
add CackeKey
add Cache to ApiTraverser
cache the get() method

add test for "get sets cache"
flush cache on write operations
use `deleteAll` for cache expiration

It looks like `flushAll` is a nuke-from-orbit operation which may have
effects beyond the namespace. `deleteAll`, on the other hand,
specifically looks for cache entries in our namespace.
eliminate repeated method calls
test the class of cached values
rename test
Merge pull request #17 from easybiblabs/topics/cache-results

Cache responses