Add u5 for fetching Go package importers via godoc.org API.

I may turn this into a fully fleshed out package similar to
github.com/google/go-github, but only when I/others have a use for it.
No point in creating something no one uses. It's also possible it
already exists, I haven't really looked (if it does exist, it should be
added to gddo README.md).
May want to look into conditional request/caching if
possible/needed/wanted.
Thank you @garyburd.