Use `dotenv-rails`

`dotenv-rails` enables local environment settings.
Use `bundle exec` for running clock process

Proposed upstream as https://github.com/harrystech/cronut/pull/4
Add binstubs
Allow other time zone with `CRONUT_BASE_TIME_ZONE`

For groups whose offices are not in EST, this change allows for the
specification of an alternate time zone to use for scheduling through an
environment variable, `CRONUT_BASE_TIME_ZONE`.

Other changes:
* Using `strftime`'s `%Z` parameter to abbreviate time zones. This does result
  in the standard times being `EDT` instead of `EST`, but does allow for
  interface and alert output to use preferred time over a hard-wired zone.
Make Clockwork config depend on TIME_ZONE instead of env variable
Fix README to be more accurate
Reduce Heroku warnings

* Declare explicit Ruby version
* Use `rails_12factor` gem to avoid plugin injection
Allow use of personalized from address for mailer

Admins can set `CRONUT_MAILER_EMAIL` to override the from address with one that
matches their organization rather than use the default `cronut@harrys.com`.