Add go-wrapper support for Go 1.4 Custom Import Paths, relegating .godir support legacy and "second fiddle"

Fixes #29

I haven't run `update.sh` yet in the spirit of making this easier to review. :+1:
(the goal was to avoid the `cat` altogether if the file isn't even there, but I guess it doesn't really matter much -- I can change it if you think that's easier to read/understand)