EXP-25. Fixes EntityId's toJavaEntityId method.
EXP-26. Adds a class which can run a Scala shell.

This commit adds ExpressShell, which can be used to run a Scala
REPL from the command line. This is the first step in providing
a custom REPL for KijiExpress.

A few simple tests are included, but more complex ones have been left
pending due to initial implementation difficulties.

Reviewed by: robertchu