teal_13_17_acu134044_support_mingw

@szmyd added logic to Rakefile and gemspec such that rake gem will generate two .gem files in the pkg directory; one for Linux and one for mingw.
this works regardless of the current platform (i.e. works the same on Windows or Linux).
this is necessary because rubygems 1.8.x by default will only produce a .gem with a gemspec that lists gem dependencies for the current platform even if logic in the gemspec would indicate different gem dependencies for other platforms.