Additional fields for epics, issues, jobs, merge_requests, milestones and users

Closes #44 (closed)

Added the following fields

• epics.web_url
• issues.epic_id
• jobs.web_url
• merge_requests.reviewers
• merge_requests.has_conflicts
• merge_requests.squash_commit_sha
• milestones.web_url
• milestones.expired
• users.email

To support the users.email I moved the project specific users endpoint to the global users endpoint, as the global users endpoint + an admin token is the only way to get the user email addresses (please correct me if I'm wrong).