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
Inventory option to specify the inventory file to use
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
If the --inventory  option is not used, use the default host list
Merge pull request #17 from iambocai/master

add log and step feature
Merge branch 'master' into feature/inventory-option
Blank lines
Merge pull request #18 from MiLk/feature/inventory-option

Inventory option to specify the inventory file to use
Typo
Merge pull request #21 from MiLk/fix/typo

Typo
Add pip installation instructions to README

I forget pip's VCS URI syntax every damned time. You'd really think that
would be summarized in `pip help install` instead of forcing you to go
look up the web docs...
Merge pull request #22 from ches/docs

Add pip installation instructions to README
added license fixes #24