Add cross-compilation to Travis

As noted in https://github.com/docker/swarm/issues/209#issuecomment-68330145, this excludes Windows for now (`api/server.go:29: undefined: syscall.Umask`), and also excludes Go 1.4 for now (https://github.com/moovweb/gvm/pull/116), but does include everything necessary to reenable them later without much fuss (once the issues with each are respectively resolved).
4:07 PM <vieux> tianon: is the name `TRAVIS_GLOBAL_WTF` important ?
4:07 PM <vieux> or can it be anything else ?
4:07 PM <tianon> nope, it can be anything
4:08 PM <tianon> that was just the name I came up with when frustrated that I couldn't just say "run these scripts outside the build matrix"
4:08 PM <vieux> ok I see