Set correct defaults in new and merged catalogs

[tricoder42]

Fixes #199, #212, #222

New catalog:

• translation: Message ID is copied as a translation for sourceLocale for messages with auto-generated IDs. Empty otherwise.
• defaults: Auto-generated message for custom IDs. Empty otherwise.

Merged catalog:

• translation is updated for sourceLocale if wasn't changed from defaults.

Minimal format:

• always uses either translation or message defaults. This is how lingui compile resolves message in lingui format.

Known drawbacks:

• when using custom message IDs and minimal format, once catalog for sourceLocale is generated translations must be always edited manually. Even when default translation in source code is updated, the translation in catalog won't be updated automatically because there's no way to distinguish default translations from custom ones.