0.4.1 - EventPlugin, createRepeatedField - DO NOT USE

• added FieldType class. It turned out that FieldType is a
  commonly used name, even in .proto files. This is renamed to
  PbFieldType in 0.4.2, so use that release instead.
• Added support for observing field changes.
  For now, this can only be enabled by using a mixin to override
  the eventPlugin getter.
• Removed optional third parameter from setField().
  It was only intended for internal use, and could be used to
  defeat type checks on fields.
• clearExtension() removes the value and extension in all cases.
  (Before, the extension would be kept and the list cleared
  for repeated fields.)
• Upcoming: clearField() will require its argument to be a known
  tag number (which could be an extension). For now, this is only
  enforced when a mixin provides an eventPlugin.