DockerImage::loadFromSlashNotation

Summary:
API users aren't expected to split themselves their acme/foo image
notation in "acme" and "foo", it's more convenient to provide a static
constructor to do it directly in DockerImage abstract class.

Test Plan: New unit tests in DockerHubImage, which implements DockerImage.

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D637