Summarizer should include copyright statements at the top of poms

Per [the determining declared license documentation](https://github.com/clearlydefined/service/blob/master/docs/determining-declared-license.md), when the service summarizer determines the declared license of a maven component:

* The crawler downloads the maven artifact from maven.org
* The crawler then extracts all pom files from the artifact
* And then merges the poms (including all the licenses defined in the poms)
* The ClearlyDefined summarizer parses the merged poms and sets the declared license(s) based on that data

The summarizer should also parse copyright statements at the top of pom files, rather than just what is between the tags.

@capfei - do you have any examples of curations to maven components that were because a copyright statement at the top of a pom file was missed?