Support for creating a store-service for Storm platform

To be able to do leftJoin on a store we need the same object to serve as a store and a service. This is already [supported in the Scalding Platform](https://github.com/twitter/summingbird/blob/1b630690a8efe554ab3fabc9b60965c2c95e2e83/summingbird-scalding/src/main/scala/com/twitter/summingbird/scalding/Service.scala#L46). This adds a trait to the Storm Platform that is of both P#Store and P#Service types. This can be used in jobs that use leftJoin against store. Users in their jobs can call:

```
Storm.storeService(mergeableStore, serviceStore)(batcher)
```
where the mergeableStore is the store for aggregation (delta store) and serviceStore is the store that we use as a service (can be delta store or the total store). For read-modify-write behavior the serviceStore should include the delta store, i.e. serviceStore can be a client store that wraps the delta mergeableStore or a client store that wraps the delta mergeableStore and the offline store (i.e. the total store). 
1. ClientMergeable could use composeKeyMapping to drop the batch and get a ReadableStore. But I think the ClientMergeable is really only needed when sumByKey output is consumed, not for the joining on store case.

2. Can you add that function you have there to `object ClientStore`? Something like?

```scala
def storeAndService[K, V](m: MergeableStore[(K, BatchID), V], batchesToKeep: Int)(implicit b: Batcher): CombinedServciceStoreFactory[K, V] =
// you can get the semigroup/monoid from the MergeableStore, right?
```