Refactoring REPL.

Mostly minor stuff, but two slightly bigger things

1. take advantage of some scalaz-stream conveniences and
2. get rid of the `throw` on exit (which was never good, but caused a
exceptional exit after upgrading to scalaz-stream 0.5).
Merge remote-tracking branch 'upstream/master' into ready/repl-refactor

Conflicts:
	src/main/scala/slamdata/engine/repl/repl.scala
Merge pull request #303 from sellout/ready/repl-refactor

Refactoring REPL.