Improve the POM.

Fix Nick's name and declare the correct contributors, instead of the
dummy ones.
Remove the license header from the Perl file.

It's already in the POM.
Fix warnings on uninitialised values.

The component was using $file (unititialised) in some places, and
$opts{file} in others.  We make it consistent now and use $opts{file}
everywhere.

Fixes #25.
Add simple tests for the download method.

This is the most important part of the component.  Having simple tests
should prevent future breakages.
Merge pull request #66 from piojo/download-cleanups

Fix warnings and small bugs in ncm-download