Removed use of Jline in UI class. Input is read through readAndExecuteCommands() method.
Merge branch 'master' of https://github.com/cs2103aug2014-T13-2J/main

# By mervynwee
# Via mervynwee
* 'master' of https://github.com/cs2103aug2014-T13-2J/main:
  Fix the bug where if the user keeps undoing till there's no more commands and enters the command display, the program will crash.