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
* Added Docs button and `button` class.
* Refactored download button style into `.button`
* Applied color overrides for download/docs buttons.
* Pointed docs link to latest available docs.