* Supplement Slack service

Refs:
- https://docs.gitlab.com/ce/api/services.html#slack-notifications
- #142

* gitlab: move BoolValue type and associated methods to gitlab.go

* services: update slack service: add url tags, fix typo, remove pointers in properties

* Revert renaming webhook field

* Fix ListProjectContributionEvents fails with 404 error (#449)

* Rename EventsService.ListProjectContributionEvents to EventsService.ListProjectVisibleEvents to match with gitlab API changes

* Adding missing straight option in repositories compare api

* Fixing struct reference for missing straight option

Adding support for 'list-merge-requests-associated-with-a-commit' (reference: https://docs.gitlab.com/ce/api/commits.html\#list-merge-requests-associated-with-a-commit) (#447)

* Adding Group Variables support

* Fixing naming per request

* Fixing merge conflict

* Fixing merge conflict

* Adding support for group issue boards

* Cleaning up comments and renaming GetGroupIssueBoardLists() to ListGroupIssueBoardLists() to match the gitlab API spec.

* Fixing comment lengths

* Adding Project Variable Support

* Adding missing semi-colons

* listtagopts added params

* test fix + gitignore

* added commit missed functionality

* rename vars

* docs fix

* requested changes

* type mistype

Ref: https://docs.gitlab.com/ce/api/issues.html#edit-issue

Ref: https://docs.gitlab.com/ce/api/award_emoji.html#award-a-new-emoji

* add pipeline to merge request

* sha in caps

ci skip

* Fixes https://github.com/xanzy/go-gitlab/issues/407
* Add the first groups unit test

* Allow users to pass the base URL without the API info

* Makes SetBaseURL more robust by matching on apiVersionPath suffix and adds unit test (#405)

* Fix tests

* add missing members of List{,Group,Project}IssuesOptions

* go fmt

* fix golint exit status

* add comments to fix golint complaining

* make go vet verbose

* fix wrong usage of printing

* Improve the way we handle tokens

* Use Go's OAuth2 package (#397)

* Use Go's OAuth2 package

* Pass HTTP client to OAuth2 via context

* Create a basic client with username/password

This client will retrieve an oAuth token using: https://docs.gitlab.com/ce/api/oauth2.html#1-requesting-access-token

* Do not use the libraryVersion in de userAgent

As this never got updated, it did’t have much value (if any).