weboob.capabilities.base: BaseObject implements pickle methods

By default, PyYAML dumps all fields, giving poor results for BaseObject
and its fields:

  !!python/unicode paris: !!python/object:weboob.capabilities.calendar.Query
    _fields: !!python/object/apply:collections.OrderedDict
    - - - url
        - !!python/object:weboob.capabilities.base.StringField
          _creation_counter: 0
          doc: url
          types: !!python/tuple
          - &id003 !!python/name:__builtin__.unicode
          value: !!python/object:weboob.capabilities.base.NotLoadedType {}
  etc.

When pickle methods are provided, PyYAML uses them.
In this case, a BaseObject will be dumped as a plain old dict.