JAXB POM File Updates for JDK 1.7

This commit provides an update to the atom-hopper POM files to allow for clean compilation on a stock JDK 1.7 installation. This is designed to fix [issue 245](https://github.com/rackerlabs/atom-hopper/issues/245).

This commit adds a dependency on the [jaxb2-maven-plugin](http://mojo.codehaus.org/jaxb2-maven-plugin/) maven plugin. In addition, this commit adds a configuration that it compliant with what is expected of the already existing project model.

This commit has been tested on:
* Java version: 1.7.0_03, vendor: Oracle Corporation
* Java version: 1.6.0_45, vendor: Sun Microsystems Inc.