Revert "Change import path from code.google.com/p/go.net/context to golang.org/x/net/context."

This reverts commit 718390f8d6d12c45a7671f4b6dd43b7060e31ce2.
Revert "Revert "Switch to fork of go.net""

This reverts commit 00d9bc35eae8cce578844d2becf8c59950d78322.