Use bundle-plugin to allow Dropbox SDK to be deployed to OSGi containers

I added the maven-bundle-plugin [1] to the pom.xml to be able to deploy the Dropbox SDK to a OSGi container. The bundle plugin adds the Bundle-* and Export-Packages Attributes to the manifest automatically.

[1] http://felix.apache.org/site/apache-felix-maven-bundle-plugin-bnd.html.