Add methods for finding, creating, updating and deleting users

I rebased the work of @zpenzeli in pull request #3 which was not mergeable and had some smaller issues, added documentation for it and added methods for creating, updating and deleting users.

For the documentation, if have tried to reuse the official documentation of the REST API.

The pull requests includes unit tests for the UsersJsonParser and the UserInputJsonGenerator, but no integration tests yet. For the findUsers methods, integration tests might be reasonable, the create, update and delete methods are really straightforward so that I think they don't need any tests.

Please tell me what you think about it and if you like it, what further work might be done to get this merged.