The directory structure for viewing branches and code on Gitea / Gogs is
different from GitHub. This fixes that, so the Source Code link on the
gopkg webpage works, as well as the URLs to lines in the source code
included in GoDoc.

...instead of /go-{repo}/{repo}

This works better for us, as we have /writeas/go-writeas and
/snapas/go-snapas

Allows you to configure a different code hosting site besides
github.com, like one powered by Gitea or Gogs.

For details see:

https://github.com/go-fsnotify/fsnotify/issues/1

This also includes further tweaks to unify the handling of
FullVersion across the go-get and the package page rendering.

Besides enabling support for repositories with no master branch
defined, this will also make a bare "git clone" on a gopkg.in
repository create a local branch with the proper content and name.

Fixes #20.

Fixes #25.

See https://github.com/golang/gddo/pull/212 for the corresponding
change to godoc.org.

A few minor tweaks.

Closes #1.

Fixes #15.