Extract inner classes from Message into separate files; classes are now not nested.
Move message classes to subdirectory; remove 'message_' from filenames.
Merge pull request #32 from keithrbennett/restructure-message-files

Restructure message files
Merge branch 'master' into refactor-resource

Conflicts:
	lib/dnsruby.rb
Add comment to dnsruby describing CodeMapper subclasses found in code_mappers.rb.