Add release script to generate cross compiled binaries.

I'd like to start shipping precompiled releases once we arrive to a point where we are happy with the feature set.

This adds a script to generate binaries for a bunch of platforms.

Idea borrowed from http://dave.cheney.net/2013/07/09/an-introduction-to-cross-compilation-with-go-1-1

/cc @d2fn, @technoweenie