Implement Users endpoints

[cdancy]

A new UsersApi should be created along with PR's for each endpoint.

https://docs.atlassian.com/bitbucket-server/rest/6.0.0/bitbucket-rest.html#idp362

[ajammil]

Hello Mr @cdancy,
I almost did the first endpoint, but I need feedback.
Also in the Mock test for the UserApi, I disabled the assertSent(..) function as it gave me errors, Could you tell me what I did wrong?
Thank you in advance.
https://github.com/ajammil/bitbucket-rest/tree/UserApi-Development
Test link
https://travis-ci.com/ajammil/bitbucket-rest/builds/106120272

[cdancy]

Feel free to open a pull request to get feedback earlier as well.

I'll take a look at things in more depth shortly...