use 'cl-lib' macros instead of cl.el

Using `cl.el` macros causes byte-compile warnings as bellow.

```
Compiling file /home/syohei/tmp/docean.el/docean.el at Wed Nov 26 11:33:00 2014
Entering directory `/home/syohei/tmp/docean.el/'
docean.el:134:9:Warning: reference to free variable `docean-request'
docean.el:135:25:Warning: reference to free variable `&key'
docean.el:149:26:Warning: reference to free variable `headers'
docean.el:152:35:Warning: reference to free variable `endpoint'
docean.el:153:20:Warning: reference to free variable `type'
docean.el:157:20:Warning: reference to free variable `data'
docean.el:155:22:Warning: reference to free variable `params'
docean.el:158:14:Warning: reference to free variable `parser'
docean.el:159:23:Warning: reference to free variable `success'
docean.el:160:21:Warning: reference to free variable `error'
docean.el:161:14:Warning: reference to free variable `timeout'
docean.el:162:14:Warning: reference to free variable `status-code'
docean.el:163:14:Warning: reference to free variable `sync'
docean.el:165:9:Warning: reference to free variable `docean-default-callback'
docean.el:165:44:Warning: reference to free variable `response'
docean.el:165:53:Warning: reference to free variable `error-thrown'
docean.el:165:66:Warning: reference to free variable `&allow-other-keys'
docean.el:196:9:Warning: reference to free variable `docean-action-callback'
docean.el:315:1:Warning: Unused lexical variable `response'
docean.el:321:33:Warning: reference to free variable `page'
docean.el:322:33:Warning: reference to free variable `per-page'
docean.el:364:1:Warning: Unused lexical argument `response'
docean.el:364:1:Warning: Unused lexical variable `response'

In end of data:
docean.el:424:1:Warning: the following functions are not known to be defined: defun*,
    endpoint, type, params, data, parser, success, headers, timeout,
    sync, status-code, &key, docean-droplets, docean-request, page,
    per-page, function*, docean-actions
```