Follow and "deference" license URLs

[pombredanne]

In Maven POMs and other structure formats we can have explicit license URLs such as in:

 <licenses>
    <license>
      <name>BSD</name>
      <url>https://github.com/googleapis/api-common-java/blob/master/LICENSE</url>
    </license>
  </licenses>

We should have an option to follow these URLs and use a "detection" to handle the merge with the remote license.