Improvements and fixes
- Improved validation during encode and decode "purl" string
- Fixed CPAN repository URL
- FIX Qualifiers are case sensitive (#4)
- FIX PURLs containing multiple namespaces segments parse incorrectly (#5)
- FIX Incorrect parsing of PURLs that begin with "pkg:/" (#6)
- Improved "t/99-official-purl-test-suite.t" test