Don't inherit from Exporter

Instead, import only its 'import' method. This requires Exporter 5.57
bundled with perl 5.8.3, but also available on CPAN for older perls.
Add basic .gitignore
Let EUMM manage the MANIFEST using exclusion in MANIFEST.SKIP
Allow only ASCII digits for level arg detection
Ignore the package element in caller() result
Add t/basic.t
Refactor caller level extraction
Refactor return value building to avoid intermediate storage
Test with call level 1
$VERSION as string to avoid surprises

Reference:
http://www.dagolden.com/index.php/369/version-numbers-should-be-boring/
Makefile.PL: cleanup spaces at end of lines