Create ~/.ssh directory when using private key

Previously, if the ${HOME}/.ssh directory did not already exist when
attempting to checkout a Git repository via SSH, the
checkout edx-platform repo and checkout theme tasks would fail.

This issue doesn't appear when checking out code as the edxapp user
(default: presumably this directory is created elsewhere), but will
surface if run as another user who lacks a ${HOME}/.ssh directory.

The code now looks before it leaps.
Add stv to AUTHORS
Merge pull request #3 from Stanford-Online/o/stv/ssh

Create ~/.ssh directory when using private key