Added getOptionalProject() with statistics option and Stream methods for all gets that return multiple items (#283).

* Add support for resolve_outdated_diff_discussions in Project
* Add support for initialize_with_readme in Project

1) Added a method to get languages used in a project.
2) Added LicensesApi & MarkdownApi classes and methods

Added getProject() methods which return a single project with statistics and methods for retrieving project archives with a specified format. (#200)

* Load forks of a project with Pager
* Load comments of a commit with Pager
* Load merge request changes
* Load merge request participants.
* Method to request repository contributors
* getMergeRequestChanges method needs list of diffs
* Fixed getFile method does not handle spaces in path correctly

* project archiving and unarchiving
* expected response for archive and unarchive fixed
* remove unnecessary preconditions for archiving test

* Get project users (not members)
https://docs.gitlab.com/ce/api/projects.html#get-project-users

* Removed wildcard imports