Upgrade doxygen_utility to DoxygenUtility plugin.

Add:
	vim/scripts/(doxygen_utility.vim -> DoxygenUtility/)
		-Rename `doxygen_utility.vim` to DoxygenUtility, and convert it from a
		one-file script to a full-blown plugin.

	vim/bundle/DoxygenUtility
		-Add symlink to the `DoxygenUtility` plugin, to allow sourcing by
		`pathogen`.