facebook plugin
(facebook) refresh token on each operation function call
fix bug that failed refresh token will lead to not authed
move is_authed to private _is_authed
fix bug on access_token refresh and paging
(facebook) auth via web
reply to a feed status, and fix unicode issues on update status
Merge branch 'master' of github.com:fqj1994/snsapi into dev
text_length_limit in full new_channel entry; some code cleaning
FB plugin. Change convention:

   * 'app_key' + 'app_secret'
   * 'expires_in'
   * auth_info
   * two stage auth flow. (upper layers can substitute
   `request_url` and `fetch_code` to implement different senarios)
fix 'expires' and save longer token if refreshed:

   * FB call it 'expires' in the long token URL;
   call it 'expires_in' in the short token URL.
   * I'm wordless...