add log and step feature

1. Add ‘—step’ option like which in ansible-playbook, user can can
confirm before run command
2. Notice user when user runs serial without args
3. Always show selected host numbers on prompt
4. Add background log(via callbacks.display)
5. Beautify the output format
fix problems mentioned in #17

1. avoid logging confirm answers into completion list
2. regard `cd  `(no args) as "select no host" in all functions.(and `cd /` for all)
fix: comfirm crash when list_hosts return str item

ansible list_hosts method may return str items (we expect Host instences),fix it
Merge pull request #17 from iambocai/master

add log and step feature