Implement clone command to clone a PDK project

[obbardc]

At the moment it's a difficult process for a user to clone a project, let's make it a bit easier!

This should be a standard git clone of the project files (including the etc/channels.xml?).
It should also set this up as a git remote called origin for future pulls.

The pdk download and pdk repogen commands will then download debs from our repo when needed.