Import all pull request commit authors.

Instead of Curry importing just unauthorized or unknown commit authors,
import all commit authors in a pull request and set the
authorized_to_contribute attribute on the Curry::CommitAuthor
appropriately. This is a more comprehensive way of tracking
authorization state for commit authors.
Rename method: import_{unauthorized_,}commit_authors
Add spec to protect against btm-like scenarios
Merge pull request #543 from opscode/curry-imports-all-commit-authors

Curry imports all commit authors