0.3.1

- [X] Backlog
- [X] Version
- [X] Avoid duplication in declaring the org-trello version
- [X] Add the important information about emacs 24.3+ (org-trello is only supported from this version range)
- [X] Clean the code regarding the old checklist/item entities (with stars) - https://github.com/ardumont/org-trello/issues/105
- [X] Sometimes, multiple cards are created more than once. Improve the proxy policy regarding this - Unable to reproduce so do nothing.
- [X] Release notes