Fix replConfig merging and evaluate values in Config.fromHadoop

Adds `Config.defaultFrom` which will correctly merge Config.default with existing values in JobConf (keys besides "io.serializations" can be added as needed), and use this in `replConfig` so we pick up the correct values in Hadoop mode.

Also had to fix ReplTest because the Execution[T] changes had made it so it was only ever running in Local mode. Because ReplImplicits.mode is a mutable var which gets picked up dynamically, the repl tests have to be run sequentially (for now).
Just kidding, it's totally intuitive:

~~~scala
scalding> config += ("foo" -> "1")
scalding> config.get("foo")
Option[String] = Some(1)
~~~