Add jline2 to SBT library dependencies.

JLine is a Java library for handling console input. It is similar in
functionality to BSD editline and GNU readline:
https://github.com/jline/jline2
Don't show password in plain text.

Use jline's ConsoleReader.readLine(Character) which reads the next line with
the specified character mask.
Add tab completion of action names to GameClientConsole.

Use jline's Completer to support tab completion of action names.