Implement source_module attribute
Rename source_module to source_file

Also add a test for using a POD file as source
Automatically use pod file for main module

If the dist contains a .pod file with the same basename as the main
module file, the pod file will be used instead by default. Setting the
source_file option explicitly will override this.
Expand test suite coverage

The test suite now tests a variety of possible scenarios involving the
source_file attribute.
Merge pull request #3 from DarwinAwardWinner/master

Implement source_module attribute