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.
Use `bundle exec` when executing clockwork

Starting the Cronut server in development if you have a newer version of
clockwork on your system than Cronut is using results in an error unless you use
`bundle exec foreman start`, which then requires you to have foreman in your
`Gemfile`. This change treats `clockwork` the same way `rails server` gets
handled when the Procfile processes are started.
Merge pull request #3 from cortexintel/allow-other-time-zones

Allow other time zone with `CRONUT_BASE_TIME_ZONE`
Make Clockwork config depend on TIME_ZONE instead of env variable
Merge pull request #4 from cortexintel/use-bundle-exec-for-clockwork

Use `bundle exec` when executing clockwork
Fix README to be more accurate
Merge branch 'master' of github.com:harrystech/cronut
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`.