Add an Executor to run flows without a Job

This is based on #914 The goal here is to make it easy to take a function that produces a TypedPipe, and to run that.

Next, we should have a methods that write `TypedPipe[T]` or `ValuePipe[T]` to the temp directory, then reads that in the Future to get a `Iterable[T]` or `T` respectively.

With the above, you can easily write jobs that loop that does some ML algorithm, for instance.
https://github.com/cwensel/cascading/blob/f152df983a70b342dd77c02b84f82fd5026b5c0c/cascading-core/src/main/java/cascading/property/AppProps.java#L65

Looks like we need to special case that one in Config. Dammit.