goinstall: allow packages from launchpad.net/~user branches.
goinstall: allow packages from launchpad.net/~user branches.

The permitted filename characters should include ~ to allow
the names of user-owned branches in Launchpad.

R=golang-dev, rsc, n13m3y3r, gustavo
CC=golang-dev, gustavo.niemeyer
http://codereview.appspot.com/5280052

Committer: Gustavo Niemeyer <beac5f8171fcde5e2ec734cc5d25d03e7362e8de@niemeyer.net>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go