Improve pom.xml support

[andrew]

• Check dependencies inside < dependencyManagement> if present: https://github.com/ctubbsii/accumulo/blob/master/pom.xml#L157
• Use attributes from <parent> for replacing values like $project.groupId: https://github.com/ctubbsii/accumulo/blob/master/pom.xml#L121 and https://github.com/GoogleCloudPlatform/gcloud-java/blob/master/gcloud-java/pom.xml