add org-mode dependency to org2blog

This change fixes an error about the missing ox-html library. This library is located within el-get/org-mode/lisp/ox-html.elc when I have org-mode installed (with el-get). Without org-mode, org2blog doesn't fully load.
Merge pull request #1885 from thiagowfx/patch-3

add org-mode dependency to org2blog