Rewriting IANA registry parsing.

- The IANA registry format changed layout in (X)HTML, but an XML version
  was made available that is far easier to parse and provides more
  useful information.
- Adding #xrefs/#xrefs_urls to MIME::Type to reflect the more useful
  data available in the new IANA registry XML format. Over time,
  #references and #urls will be views on the #xrefs data.
Removing an unused Rakefile task.
Summarizing the impending mime-types 2.1 release.
Adding support for the Apache list.

Resolves #38.