Add clock mock plumbing throughout the project

[pzduniak]

Hey, it appears that the repo included github.com/jmhodges/clock as a dependency at some point, but it was removed (#231).

As I was trying to integrate pebble into our integration tests, I ran into the issue of the lack of the clock mocks, which was a dealbreaker. I reintroduced the dependency here, unfortunately it breaks the external API and it's probably not done the way you, the maintainers, would approach it. I'm open to spending some time on fine tuning this PR to better match the rest of the codebase stylistically. This was the bare minimum of changes I had to make to make it all work.