Don't blindly source go env.

Close #9
ZSH compat update on crosscompile.bash

Zsh doesn't split strings on blanks like bash with $IFS set to `$' \n\t'`. 
`shwordsplit` is a workaround which doesn't require to convert your long "bash-iterable" strings to arrays.
Merge pull request #10 from jeromenerf/patch-1

ZSH compat update on crosscompile.bash
Update crosscompile.bash

My previous PR wasn't as good and functional as testing against `type`.
Merge pull request #11 from jeromenerf/patch-2

Update crosscompile.bash
modify for zsh sourcing
don't clutter shell variables or overwrite existing $PLATFORM
define functions only for platforms that are available
incorporate updates from davecheney plus new features
go-build-all with build arguments