Use mails_viewer in development env

Ease email preview and accounts activation
async mailing
#171: add delayed job settings
update readme, add code climate badges. close #177
Merge branch 'feature/mails-viewer' of github.com:doitian/19wu into doitian-feature/mails-viewer
update setup rake, add e-mail receive tip. #164
Merge branch 'feature/async-mailing' of github.com:windduke/19wu into windduke-feature/async-mailing
add guard-delayed gem. #180
add guard-rails gem. #180