Created models for diff, issue, and milestone as well as an enum for response codes.
Working on the initWithJSON methods in the models and aligning the properties with those in the api docs.
More progress on initWithJSON methods in the models.
Merge branch 'jeff-dev' of github.com:Indatus/objc-gitlab-api into jon-dev
add method declaration for login
add unit test class for GLGitlab utility