introduces the 'meteor admin get-machine' command; changes publish not to publish binary builds

This commit does the following:
- Introduces the get-machine command. This command contacts the build farm server
 gets back a machine reservation and then opens    a secure shell to the machine (Alternatively,
 you can ask for a json). This also involved factoring out some   commands to deal with authenticated
 ddp from package-client into a more general auth-client.

- No longer publish binary builds in publish or publish-release; instead give the user a warning
to run get-machine and then publish-for-arch. Someone could ignore this: --existing-version and
publish-for-arch both publish binary builds, but you need to be    at least somewhat familiar with
what you are doing to run them.      Hopefully, you are running them  from a certified build machine,   but
if you are not,    then, well, it    is your    package.

Stuff remaining:
  - We are going to have a url to external documentation, but I haven't written it yet.
  - We are currently talking to   the test-build server, instead of the build server, so mac doesn't
work.
(Neither of those changes require significant tool changes)