Adding an "ssh" client for conduit

Summary: ..."ssh" is in quotes 'cuz this is step 1 and there's no ssh in sight
at the moment.

Test Plan:
ran api.php PHID-USER-xee4ju2teq7mflitwfcs differential.query a few times...

• tried valid input, it worked!
• tried bad input, it worked in that it failed and told me so!

ran api.php crap_user differential.query a few times...

• verified error message with respect to crap_user

ran api.php PHID-USER-xee4ju2teq7mflitwfcs crap_method a few times...

• verified error message with respect to crap_method

visited http://phabricator.dev/conduit/method/differential.query a few times...

• tried valid input, it worked!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Maniphest Tasks: T550

Differential Revision: https://secure.phabricator.com/D1357