Add method to retrieve merge request changes

Add missing fields to GitlabProject

Add Method to return Merge Request by iid

Add system, upvote, downvote to GitlabNote

Add Method to get a single Merge Request Note

Add Method to Update Merge Request Notes

Add Method to Update Merge Request

Correct name of GitlabAPI Test

Fixes #78

Add method to return API User

Add methods for the new commit status API

support for ssl verification in webhook

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minor cosmetic cleanups

Support OAuth access_token

Add system hook API support

Add API method for get SSH key with user information by ID of an SSH key: getSSHKey

API function was introduced by PR: https://github.com/gitlabhq/gitlabhq/pull/9591

Compilation fix

added methods:

For OAuth the uri query parameter should be named access_token instead of
private_token. To support both a enum TokenType is introduced and should
be used to select which token type to be used for the API access.

For backwards compatibility TokenType#PRIVATE_TOKEN will be used by
default.
Signed-off-by: Andreas Häber <andreas.haber@intele.com>