Skill Proficiency Levels

Extend hr.skill to support employee proficiency grading.
(see https://code.launchpad.net/~savoirfairelinux-openerp/openerp-hr/7.0-skill/+merge/195302)

Features
=======================
* Each Skill can have a proficiency grading scale.
  E.g: "Python programming" is assessed in a "A - F" grading scale.

* A proficieny grading scale has a identification title and a list of possible values.
  E.g.: "A - F" contains "A-Excellent", "B-Good", "C-Sufficient", ...

* An Employee can have many Skills, and each can have an assessed Proficiency Level.
  E.g. "Raphael" knows "Python programming" at proficeincy "A-Excellent"

Implementation outline:
=======================
Currently, an employee either has or does not have a Skill:
  hr.employee:

Proposed changes:

* New "hr.skill.proficiency.level" columns:
    name,
    description,
    proficiency_id (-> hr.skill.proficiency),
    level (integer)

* New "hr.skill.proficiency:", columns :
    name,
    proficiency_level_ids (->N: hr.skill.proficiency.level),
    sequence

* Modify "hr.skill":. Columns:
    Add "proficiency_id" (-> hr.skill.proficiency), for skill's applicable level list
    Add "ref" (char), can be useful to reference the skill in custom code or action rules
    Change employee_ids to point at "hr.employee.skill"

 * Create "hr.employee.skill". Columns:
    employee_id,
    skill_id,
    proficiency_level_id,

   Coud also be useful to also have:
    proficiency_id (related field?)
    date_start, date_end
    note (text)

* Modify "hr.employee":
    Modify:
      'skill_ids': fields.many2many('hr.skill', 'skill_employee_rel', 'employee_id', 'skill_id', 'Skills'),
    To:
      'skill_ids': fields.one2many('hr.employee.skill', 'employee_id', 'Skills'),