v0.14.0 release

• Fixed a bug where a space was missing in the exec_cli method argument list.
  Credit: @missedone
• Refactored create/update jobs by introducing create_or_update methods.
  Credit: @riywo
• Enhancement to crumb processing - auto detect the change of crumb setting and
  do proper exception handling. Credit: dougforpress
• Added a User class which will handle jenkins users related functions.
  Credit: dougforpres
• Added a method Job#poll which will poll for SCM changes programatically.
• Added a shortcut method System#restart! for force restart.