Add image action pull_if_missing and chg pull to match cli

[jhulten]

• Add the pull_if_missing action to the image LWRP with the current functionality
• Modify the existing pull action to check the hash of the image before and after to determine if the resource updated (and notifications should be triggered)
• Add docker_inspect method to the helpers library

Addresses #54

[bflad]

Well, I'll give it an hour or two for other comments, but this looks good to me. Glad it was easy to implement with the hashing! I also agree with updating default LWRP behavior to match CLI behavior (leaving pull_if_missing as a custom action).

[jayofdoom]

+1 but I would request since this is an API change it be released only in a major release

[jhulten]

Cookbook is still 0.x.y. No sustained contract is implied.

[jayofdoom]

@jhulten by "major" release, I meant more the middle number, since that's all that's generally ever been incremented.

And while I buy the idea there's not an API contract (hell, even docker is 0.x), it's nice to make the versions matter :)

[bflad]

I'll release this as 0.28.0 when I merge it shortly. I'll only increment to 1.0 when Docker does (most likely).

[bflad]

Merging now and will release after fixing up Rubocop and some additional testing.