Refactor Curry::PullRequestAnnotator

:fork_and_knife: 

Instead of putting annotation actions in the annotator, we split them out into classes for each action. It's probably easier to follow what's going on by looking at the [new annotator](https://github.com/opscode/supermarket/blob/factor-curry-annotations/app/models/curry/pull_request_annotator.rb) before looking through the [Files changed](https://github.com/opscode/supermarket/pull/696/files).

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The purpose of this refactoring is to make it easy to separate Curry's annotation decision-making process from its annotation-making process. One reason we might want to make these phases distinct is so that we can test them independently. This will be especially handy for exhaustively testing how Curry makes decisions without leaning on VCR and brittle assumptions about hard-coded GitHub repos.