Update to new GoPackage API.

It turned out figuring out if a Go package is under vcs was quite slow
(for lots of packages), so it should only be done when necessary.
Add optional "short" flag, to filter out uninteresting packages.

Thanks to @jcvernaleo for initial implementation.
Make "short" the default, rename to "all". Add "plumbing" flag for scripts.
Update Godeps.
Update Godeps to latest.
Merge pull request #10 from shurcooL/add-short-flag

Change default behavior to list only noteworthy packages; add "all" flag for old behavior. Add "plumbing" flag for scripts.
Fix inconsistency in usage example.