Import cl-lib for cl-case

I got following byte compile warnings on Emacs development version.

```
Entering directory `/home/syohei/tmp/gomi/orgbox/'

In orgbox:
orgbox.el:100:4:Warning: `108' is a malformed function
orgbox.el:100:4:Warning: `101' is a malformed function
orgbox.el:100:4:Warning: `116' is a malformed function
orgbox.el:100:4:Warning: `119' is a malformed function
orgbox.el:100:4:Warning: `110' is a malformed function
orgbox.el:100:4:Warning: `105' is a malformed function
orgbox.el:100:4:Warning: `115' is a malformed function
orgbox.el:100:4:Warning: `112' is a malformed function
orgbox.el:100:4:Warning: `113' is a malformed function

In end of data:
orgbox.el:127:1:Warning: the function `cl-case' might not be defined at
    runtime.
orgbox.el:127:1:Warning: the function `otherwise' is not known to be defined.

```