Skill Api: Change query parameter from 'queued' to 'filter'

This will allow to take various values :

   - queued:  filter like the former `queued` parameter
   - learned: fitler every unknown skills (not yet injected)
   - none:    do not filter

Default value is `learned`
Skill Api: Update documentation and tests

- Add an 'unknown' value in filter query parameters
- Replace 'none' value in filter by 'all'
- Update tests
- Update backend
Skill Api: sort result by group name
Skill Api: merge SkillQueue with Skill resources
Skill Api: update skill-tree directive to use the new API

- Add a preference dropdown menu to show learned, unknown or all skills
Merge pull request #10 from aegypius/skill-api-filtering

Skill Api: Change query parameter from 'queued' to 'filter'