Fixes BatchedStore interval calculation

There was a bug in refactoring and interval calculation in BatchedStore. This fixes it and simplifies the interval calculation. Also fixes the Scalding unit tests to use batch cover in input space instead of store space. Fixes #567
Can you add:
```scala
package object scalding {
// bunch of stuff:
def failure(msg: String): Try[Nothing] = Left(List(msg))
def success[T](t: T): Try[T] = Right(t) 
}
```
where `Try` here is our `Either[List[String], T]` type that we used before converting to scala 2.9. If you do this, we can try to replace all the code we have creating these poor-person Try's and eventually just replace with scala.util.Try.

Also, it will clean up this code.