Export project functionality

This is a MR for the export functionality of https://gitlab.com/gitlab-org/gitlab-ce/issues/3050, which adds the ability to export single projects.

• members
• DB data
  • issues
  • issue comments
  • merge requests
  • merge request diff
  • merge request comments
  • labels
  • milestones
  • snippets
  • releases
  • events
  • commit statuses
  • CI builds
• File system data
  • Git repository
  • wiki
  • uploads
  • CI build traces
  • CI build artifacts
  • LFS objects
• DB configuration
  • services
  • web hooks
  • protected branches
  • deploy keys
  • CI variables
  • CI triggers