Return revision metadata instead of just revisionKey

[achambers]

This PR slightly changes how this plugin works. Previous to this PR, the plugin would simply return a revision key which would be merged into the deployment context object.

Now, with this PR, the plugin returns an object which can contain any arbitrary meta data about the deploy revision. One proviso is that it must contain a property called revisionKey.

The following PR's need to be merged once this one has been:

• [x] https://github.com/zapnito/ember-cli-deploy-redis/pull/27
• [x] https://github.com/lukemelia/ember-cli-deploy-lightning-pack/pull/4
• [x] https://github.com/LevelbossMike/ember-cli-deploy-s3-index/pull/1
• [x] https://github.com/dschmidt/ember-cli-deploy-cp/pull/2

Closes #14

[achambers]

@LevelbossMike, you will need to make a change to your ember-cli-deploy-s3-index repo to come in to line with this PR.