helphelp should process sources on as-needed basis

[roman-neuhauser]

waiting for mogrify to resize all the images and Maruku to transform all .md files when only one file changed since last build is highly annoying, not to mention wasteful (think green, save on electricity bills, spare the laptop battery!).

[cornelius]

Pull request #11 addresses the image generation.

Generating the html files is a different and more complex thing, because all markdown files can affect all other ones, because they share the table of content. So calculating what change needs a regeneration of which file is less easy. One approach would be to only regenerate one file, when its markdown changed, but regenerate all other files as well, when it's a new file, the file was moved, or the title of the file changed. All these changes could affect other files as well.