This puts all images in doc/images/ and references them via
http://nodejs.org/images/.
Any complaints about copyright usage etc. can thus be node/joyent's
problem, rather than the problem of a downstream distribution channel.
Conflicts:
Makefile
Modify doctool.js to automatically create anchor links for
every function, for easy linking.
Include support for functions that have a <h4> level
Fixes: #1718.