User Service Plugin update to 2.0.0

- New restfull API (see readme). The plugin is backward compatible.
- Added new requst to get all or specific users
- The user is now extendible with key / values properties
- Added new requst to get roster entries of a specific users
- Added new requst to get group names of a specific users
- Added new request to add user to a group or remove a user from a group
Merge pull request #75 from Redor/openfire

User Service Plugin update to 2.0.0