POM.java: add method to get POMs on the classpath
XML: add ctor to parse XML from a URL
XML: save path to XML data when applicable

This change also adds a getPath() method to retrieve said path.
POM.java: add ctor for POM from URL
POM.java: use URL ctor where appropriate
POM.java: make POMs comparable
POM.java: add more convenience accessors

For now, we only add accessors for simple string fields.
Add unit tests for POM#compareVersions method