Abstract 'create' methods into their own trait

@johnynek proposed that  algebird could have the following trait [1]:

```scala
trait Preparer[I, O] {
  def create(in: Seq[I]): O
}
```

This would formalize the usage of ```create``` in BloomFilter, CountMinSketch, HyperLogLog, SketchMap and the proposed SketchMap1 (#251)

This class likely belongs in Aggregator.scala

[1] https://github.com/twitter/algebird/pull/251#discussion_r9051358