Be friendlier to uname on nix, rack.exe on windows
Be friendlier to uname on nix, rack.exe on windows

* Partially addresses #57 (uname for some)
* Default download for Windows is now `rack.exe` (per #53)

Note that the uname approach from #57 will not work for any ARM based installations.

/cc @carolynvs @everett-toews

ryanvz/rack,pratikmallya/rack,brint/rack,pratikmallya/rack,rackspace/rack,jarosser06/rack,DonNateR/rack,rackspace/rack,annegentle/rack,jrperritt/rack,annegentle/rack,ryanvz/rack,jrperritt/rack,dolph/rack,brint/rack,nelsnelson/rack,jarosser06/rack,nelsnelson/rack,DonNateR/rack,dolph/rack