Various improvements to Gradle build scripts

- Use gradle-bundle-plugin instead of the native OSGi plugin: the
  native OSGi plugin is deprecated in favour of the bundle
  plugin, which uses bnd to generate not only a manifest but a
  whole jar. This new plugin handles the `-include` instruction
  correctly.
- Add the SPI-Consumer and SPI-Provider headers for compatibility
  with Spi-fly
- Set encoding to UTF-8
- Make compatible with Java 1.6
- Make some dependencies transitive
- Change version to 1.0 because because no other versions
  distributed before and the component don't need to have the
  same version as the Dotify system as a whole.