gradle: Update BndPlugin to use gradle configurations and dependencies

This is the normal "gradle way" rather than directly setting classpath
variables in tasks. This will allow other gradle plugins to work better
with the bnd plugin.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #642 from bjhargrave/gradle-plugin-update

gradle: Update BndPlugin to use gradle configurations and dependencies