This change creates a scripts/ folder, moves our existing shell
script, gocover.sh, underneath it, and factors the publish logic
out of the Makefile and into the publish.sh file.
The syntax for sed differs between Mac OS X and GNU versions of
the tool, which is rather annoying. This fixes it by leveraging
the fact that `sed -i.bak ...` works, although we now need to
clean up the *.bak file left behind. (No big deal and way better
than maintaining OS-dependent logic.)
