Added mechanism to decorate GraphSail's DataStore

[alexdeleon]

Hi there,
This minor change allows one to decorate the DataStore of connections to add capabilities (e.g. caching). Here is an example how I use it:

trait CachingGraphSail[T <: KeyIndexableGraph] extends GraphSail[T] {
  import CachingGraphSail._

  override def createStore(): DataStore[T] = new DataStore[T] with CachedDataStore[T]

  trait CachedDataStore[T <: KeyIndexableGraph] extends  DataStore[T] {

    val vertexCache = CacheBuilder.newCache(classOf[Value], classOf[Vertex]).
      maxSize(10000000).
      eternal(true).
      source(super.getVertex _).
      build()

    abstract override def getVertex(value: Value): Vertex = vertexCache.get(value)
  }
}

object CachingGraphSail {
  implicit  def FunctionAsCacheSource[A,B](f: A=>B): CacheSource[A, B] = new CacheSource[A,B] {
    override def get(k: A): B = f(k)
  }
}

Best,
Alex