First achievement model

- patch for user, mailer, and issue observer
- achievement model implemented
- basic locale and mail content
- weak test
add some flavor to mailer template

- rename mail to achievement_unlocked
- correct the mailer instance methods
Merge branch 'feature/68-achievement-model' into develop
cleanup for unused file/codes
update README.md
fix BUG#275: "First Love" achievement now trigger properly
Enhanced README.md
fix: send mail only when the achievement is created
Merge branch 'release/0.1'