Add a <license/> block to groovy's POM

Maven has a nice section to describe the license the artifact is released under. For instance:

<license>
<name>The Apache Software License, Version 2.0</name>
<url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
<distribution>repo</distribution>
</license>

This was copied from Cargo: http://jira.codehaus.org/browse/CARGO-835

This is useful to projects that use license-checking tools to validate that dependencies are licensed in a manner acceptable to the project.

This is also useful for generating license reports for a project.