EXP-34. Allow users to run the shell in local or hdfs mode.

This commit allows users to run
  * `express shell` to run the shell in local mode (jobs run in
    cascading local runner).
  * `express shell --local` to do the same as above.
  * `express shell --hdfs` to run the shell in hdfs mode (jobs run on
    hadoop cluster.

This is accomplished by having the REPL pre-load Scala scripts
local-mode.scala and hdfs-mode.scala, which set Scalding's Mode.mode
to the desired mode.

Reviewed by: robertchu