Add the letter_opener gem for easier emails in development

While in development we don't need to actually
send the email, we usually want to just see it
on our local dev machine. The letter_opener gem
solves just this problem. More info at:
github.com/ryanb/letter_opener
Merge pull request #70 from skanev/emails-in-development

По-удобни имейли в development чрез letter_opener