Move "send test" to bin/mail

Summary: Like D6417. See D6413.

Test Plan:
  >>> orbital ~/devtools/phabricator $ echo "hello" | ./bin/mail send-test --to dog --cc duck --from epriestley --subject 'hey there' --attach README --tag blip --tag bloop
  Reading message body from stdin...
  Mail sent! You can view details by running this command:

      phabricator/ $ ./bin/mail show-outbound --id 6480

  >>> orbital ~/devtools/phabricator $ ./bin/mail show-outbound --id 6480
  PROPERTIES
  ID: 6480
  Status: sent
  Retry Count: 0
  Next Retry: 1373493443
  Related PHID:
  Message:

  PARAMETERS
  to: ["PHID-USER-cluwcdowc35gmperlkbi"]
  cc: ["PHID-USER-aeabc4ipqbifny3rw4ok"]
  subject: hey there
  is-html:
  is-bulk:
  mailtags: ["blip","bloop"]
  from: PHID-USER-lqiz3yd7wmk64ejugvov
  worker-task: 936543

  BODY
  hello

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6419