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).
+1 to this change. To use scalding you must, in scalding, set the ioSerializations key. It is a pain that we have all these special cases in the config. But given the way hadoop works, it seems that special cases are the only way around.