Fix up some JRuby issues

Changes:

* Fixes "Ignored" displayed instead of menu issue (#51)
* Allows `nil` for input/output
* Add `jline` to acceptable `CHARACTER_MODE` list
* Now displays paging prompt
* Raise exception on EOF in `get_line`

When testing with JRuby 1.7.3, this only leaves two types of failing tests:

* Tests which expect tabs in input (jline doesn't accept them...maybe because of tab completion?)
* Tests where "input" ends up added to "output" (but in actual use this is not a problem)