added osgi support

- in order to be able to use license3j for OSGi valid import and
  export statements need to be provided.
- note that we are using bundle plugin 2.3.7 on purpose as newer
  versions (e.g. 2.5.3) would not allow to have the License3j.java
  in the root package (and I was too lazy to figure out how to make
  it work)