Added a 'go-build-all' function to 'go build' for all platforms at once, into a folder structure

Hi Dave, 

I found your script while wanting to make my compiled Go artifacts available for all platforms, via ghpages, so I started off with this addition to your script.(It was important to write a new function just to handle the -o output-file variable). I don't know how cleanly it fits into the rest of your script, but it served its purpose for me, so here it is if you want it.

I have since gone on and re-written my function in Go itself, (using os.exec) - please take a look: [goxc](http://laher.github.com/goxc/) - this also has the 'build toolchain' option, and - my favourite - generates a markdown file with relative links to the artifacts for that build. So, I've used it for 'downloads' pages on my jekyll gh-pages sites, for goxc itself and also my small but growing list of other go projects. e.g.2 [mkdo](http://laher.github.com/mkdo).

Thanks for this work and the associated blog post. Very handy. Go's cross-platform-standalone-binary-ness is one of its big draws for me. Even if you're not interested in the pull request then I hope you enjoy the goxc thing, such as it is

Cheers
Am