BatchedReadableStore
remove batchFn and add implicit FutureCollector
Add MergeableJMapLongStore and accompanying tests

This class is very handy for testing code that interacts with a
MergeableStore[K, Long] instance.
Refactor for more idomatic Scala
set get should return None for empty sets. fix bug with set members get
a more idiomatic way to define injections
tests for sorted sets
codestyle
Remove use of AtomicLong and make thread-safe

Place a lock around the merge operation to ensure the merge
operation is thread-safe. Use a java.util.concurrent.ConcurrentHashMap
as the backing store to make get/put threadsafe
Merge branch 'develop' into redis-sorted-sets
Parallel readable store + test
Merge pull request #158 from twitter/feature/add_paralell_readable_store

Parallel readable store + test
Changes the return type of merge
bug noticed in PromiseLink
Add tests for the PromiseLink
Merge pull request #163 from twitter/merge-fix

Changes the return type of merge
Move mergeable to Semigroup
Fix tests for sparse stores
another attempt at fixing the tests
Fix up MonoidStore