Add support for getting a commit's GPG signature.

Add LocalMarkdownVersion field to Settings and UpdateSettingsOptions

Fix typo in comment

ListTodo allows for ListOptions

The GitLab API Docs are missing these options, but they are supported as
the GitLab specs show[1], and can be verified for a signed in user[2].

Including them here breaks the interface when users used assignments
without Field names.

To test the change, an PerPage list option was set in the test. This
doesn't influence the test, but does test the interface.

[1]: https://gitlab.com/gitlab-org/gitlab-ce/blob/4325986f25b7523acac49cfaefb7149208ae8553/spec/requests/api/todos_spec.rb#L39
[2]: https://gitlab.com/api/v4/todos?per_page=1

Add LocalMarkdownVersion field to Settings and UpdateSettingsOptions.
Incrementing LocalMarkdownVersion invalidates GitLab's internal
Markdown cache, which is helpful in certain situations.  See:

https://docs.gitlab.com/ce/administration/invalidate_markdown_cache.html
https://docs.gitlab.com/ce/api/settings.html#get-current-application-settings
https://docs.gitlab.com/ce/api/settings.html#change-application-settings

Add IID field to CreateIssueOptions and CreatedAt field to CreateIssueNoteOptions

Add Duration to Job resource

Add CreatedAt field to CreateIssueNoteOptions, which allows explicitly
setting the creation time assigned to the new issue note.  This allows
preserving a comment's creation time when migrating comments into
GitLab from a different issue tracker.

See https://docs.gitlab.com/ce/api/notes.html#create-new-issue-note

Add IID field to CreateIssueOptions, which allows explicitly setting
the internal ID assigned to the new issue at creation time.  This
allows preserving issue ID's when migrating issues into GitLab from a
different issue tracker.

See https://docs.gitlab.com/ce/api/issues.html#new-issue

Add support for revert a commit

Add resource label events

issues: added `in` option

implemneted get environment

tell json encoder polling_interval_multiplier is encoded as string type

add masked variable support for gitlab projects and groups

[Merge requests API] Add missing field: merge_error