This change refactors package graceful into two packages: one very well
tested package that deals with graceful shutdown of arbitrary
net.Listeners in the abstract, and one less-well-tested package that
works with the nitty-gritty details of net/http and signal handling.

This is a breaking API change for advanced users of package graceful:
the WrapConn function no longer exists. This shouldn't affect most users
or use cases.

This feature can be used in place of the pile of hacks in middleware.go,
and doesn't involve awkwardly shimming out a http.ResponseWriter. Sounds
like a win-win!

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!"