refactoring

- split feature.xml into feature.xml & feature.properties
- that's similar to about.xml & about.properties
- it's much cleaner than using CDATA in XML
- now the Feature Manifest Editor in Eclipse can be used,
  not causing reformatting ot the XML
upgrade dependencies from Eclipse 3.5 to 3.7
upgrade dependencies from Groovy-Eclipse 2.1.0 to 2.7.0

* this should be the minimum version still available for Eclipse 3.7
update build howto
merge HOWTO files