Add Python3 support to XML extractor
Use AST for python extractor

This removes the need to work around insane compat problems with python
3 and greatly simplifies the code.
Add new extract command

This replaces the use of pybabel to drive extraction.
Fix function name typo
Convert XML extractor to native plugin
Convert ZCML extractor to native plugin
Convert python extractor to native plugin
Support passing directory as parameter
Cleanup get_extractor a bit
Fix extractor lookup
Rework internal plugin API
Fix comment handling error in XML extractor
Generate timestamp in official PO-format
Small cli tweaks