Update so wro4j-core and wro4j-extensions can be used as osgi bundle

- Add maven-bundle-plugin that generates osgi metadata
- Change packaging from jar to bundle
- Changed slf4j from version 1.7.1 to 1.6.1. This wasn't necessary for
  osgi, but just convenient to resolve a dependency version conflict when
  integrating with another project that was using 1.6.1