fix #369. preserve user-typed text.

- preserve user-typed text on title,body even on change selected branch
- destructor of yobi.Files, yobi.Attachments applied (against memory leaks)
set font-family for comments in codeDiff
Merge pull request #135 from laziel/yobi refs/heads/yobi-369
fix typing error in README.md
add text center align on center-block of common css function
Merge branch 'master' of github.com:joohee/yobi into joohee-master
remove text-align css on .center-block, add text-align:center on button wrap