This is now controlled at the top level goji package: these packages are
not even compiled in when using App Engine. This is sensible, since
neither package is of any use there.

The result of this was that builds were broken on windows and app
engine.

Fixes #59.

Fixes #52.

Previously, a set of standard signals would be handled automatically via
an init() function, however that made the package difficult to use in
packages in which an HTTP server would only be spawned some of the times
(perhaps keyed on an environment variable or flag). Now, signals must be
registered manually.

By default, the top-level "goji" package automatically registers
signals with graceful, so this will result in no behavior changes for
most people.

Fixes #35.

Package graceful provides graceful shutdown support for net/http servers,
net.Listeners and net.Conns. It does this through terrible, terrible hacks, but
"oh well!"