POM.java: tweak style and formatting
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
Merge branch 'poms'

This branch adds a getAllPOMs() utility method to org.scijava.util.POM
which searches JARs on the classpath for Maven POM metadata.

It also improves the POM class in various other ways, including the
addition of a constructor that takes a URL, more accessor methods, and
making POMs naturally comparable by GAV.