Split the bnd-maven-plugin maven build up in multiple modules

This pull request makes it possible to use the latest bnd with the bnd-maven-plugin, by taking the built bndlib.jar and installing it into the local m2 repository. Then the bnd-maven-plugin uses that jar using normal maven dependencies.

The BND Maven Plugin now consists of the following modules:
* A parent module
* A module that takes the latest bndlib build and installs it in the
  local maven repo. This is to allow picking up the latest bndlib
  builds as snapshots from Maven. If we don't depend on unreleased
  bndlib builds this module is not needed any more.
* The actual bnd-maven-plugin module