Implement "Upload Artifact" build step

Summary: This implements a build step for uploading an artifact from a build machine to Phabricator. It uses SFTP so that it will work on both UNIX and Windows build machines.

Test Plan: Ran an "Upload Artifact" build against a Windows machine (with FreeSSHD installed). The artifact uploaded to Phabricator, appeared on the build view and the file contents could be viewed from Phabricator.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7582