Configure basic mail delivery

Using Mandrill on both production and development for the time being, mostly because it's more scaleable that way. (On development, test keys can be used that don't send actual mail or count against send count, but show output on Mandrill's site.)

Account environment variables need to be set in the .env file for each environment with pre-filled Mandrill account details.

Fixes #89 once functional on production.
Deploys don't work unless the Gemfile includes dalli, so now it includes dalli and everyone is happy!