Use letter_opener for ActionMailer under development environment
Add User model and base tests
Improve test cases and implement user methods
Replace faker by ffaker
Improve user model
Update gems
Change back to faker gem
Add user name and email format validation
Add password and authenticate tests
Improve tests
Finish user signup
Improve layouts
Change user introduction column type to text
Move javascript code to js file
Merge pull request #39 from xian-rubyists/dev/user-signup

Ready - 用户注册（不含Email验证）