Add go-build-all
Keep track of build failures and show them at the end #3

Keep track of errors when building over all platforms and show them at
the end after all platforms have been tried. Returns an error code if
any builds failed.
Update for Go 1.1
Merge branch 'master' of git://github.com/davecheney/golang-crosscompile
modify for zsh sourcing
don't clutter shell variables or overwrite existing $PLATFORM
define functions only for platforms that are available
incorporate updates from davecheney plus new features