$dmg used before set in tar_em_up.sh
I mistakenly download the nw.js binary before parsing the command line options. One side effect of this is that the "dmg" variable is used before getting set, which causes a (harmless error) in the OSX build.
Anyhow, the binary downloading logic should be moved below the option parser. Also, if we use "curl" to fetch the binary we can specify one less dependency in the build instructions.