By not including the 'json' gem the Request class is unable to find
JSON

     NameError:
       uninitialized constant Gitlab::Request::JSON
add 'to_hash' method for ObjectifiedHash

close #26
Merge pull request #38 from b1tw1se/master

By not including the 'json' gem the Request class is unable to find JSON class

close #22, #35
Doc & test that update_merge_request can set state

Update the documentation for Gitlab.update_merge_request to say that
Gitlab.update_merge_request can change a merge request's state.

Test that Gitlab.update_merge_request Gitlab.update_merge_request
can change a merge request's state.
Add merge_request_comments to get a MR's comments

Add a method merge_request_comments to retrieve a merge request's
comments.

Document and add spec tests for the new method.
Remove extra test for update_merge_request