Mapper API refactoring (not backward compatible)

• Add a new optional keyword argument 'description' to the delay() function of a
  job. If given, the description is used as name of the queue.job record stored
  in OpenERP and displayed in the list of jobs.
• Fix: assignment of jobs to workers respect the priority of the jobs (lp:1252681)
• Pass a new parameter to listeners of 'on_record_create' ( vals: field values
  of the new record, e.g {'field_name': field_value, ...})
• Replace the list of updated fields passed to listeners of 'on_record_write'
  by a dictionary of updated field values e.g {'field_name': field_value, ...}
• Add the possibility to use 'Modifiers' functions in the 'direct
  mappings' (details in the documentation of the Mapper class)
• When a job a delayed, the job's UUID is returned by the delay() function
• Refactoring of mappers. Much details here:
  https://code.launchpad.net/~openerp-connector-core-editors/openerp-connector/7.0-connector-mapper-refactor/+merge/194485